When no configuration is enabled, a message was supposed to be shown after startup. Now it actually shows up.
Several configurations were missing from the "Configurations" folder.

Interface: you can click on X,Y map coordinates in the right-bottom corner of interface to open "Go to coordinates" window. 

Configurations: increased game configuration loading speed (in previous builds it took ~650 ms. to load a single game configuration, now it takes ~120 ms. to load all 64 of them). As a side effect, New\Open Map Options, Map Options and Game Configurations windows are now opened noticeably faster. The editor starts up a bit faster as well.
Configurations: all 64 game configuration are now available by default.
Game Configurations window: game configurations can now be disabled. This setting is mostly cosmetic. When a game configuration is disabled, it won't be shown in "game configuration" dropdowns in New\Open Map Options and Map Options windows. If a map's .dbs file specifies a disabled configuration, it will be picked as a map configuration anyway.
Linedefs mode: vertex insert preview logic used Highlight range instead of Stitch range (which is used when draw mode engages).
Visual mode: double-sided middle textures were not selected when using "Select" action with "with same texture" modifier.
Textures: some optimizations in patch blending code.
ZDoom ACS script configuration: added definitions for StrLeft, StrMid and StrRight functions.

Map coordinates are now drawn on edit area screenshots. In classic modes, map coordinates at the cursor location are used when the cursor is inside the view and the coordinates at the center of the view are used when it is not. In visual mode, camera location is used.

Added "Go To Coordinates" action (available in "View" menu and via Ctrl-Shift-G shortcut). It will center the view on given coordinates in classic modes and place camera at given coordinates in visual mode.

Updated zdefs.acs and ZDoom_ACS.cfg. GetLineActivation and SetLineActivation action specials should work now.
Made hints group higher in Draw Grid mode's side panel.

Map Analysis mode: "Check overlapping lines" logic was broken on maps in Doom and Hexen map formats.
Map Analysis mode: "Check overlapping lines" now finds duplicate linedefs (e.g. when 2 lines have the same start and end positions).
Map Analysis mode: added "Check overlapping vertices" check. This will check if a vertex is on top of a linedef or another vertex.
Some cosmetic changes in Tools and BaseVisualMode.
Updated zspecial.acs

Visual mode: when changing height of a triangular sector all vertices of which have height offset, vertex heights will be changed instead of sector's floor/ceiling height (this allows to edit terrain without the use of vertex handles).
Visual mode: "Copy Texture", "Copy Texture Offsets" and "Copy Properties" actions now always copy properties from currently highlighted surface (previously they copied properties from the first selected surface when something was selected).
Draw Rectangle mode: nothing was drawn in some cases.
Drag Things mode: grid was not updated when grid size was changed in this mode.

Draw Settings panel: added "All" buttons to "Fill Selection with Textures" and "Remove Textures form Selection" groups
Export to .obj: fixed a crash when trying to export an unsaved map.
Texture preview are now created ~25% faster.

Sector info panel: rearranged controls a bit, so tag labels and long effect names can be displayed (more) properly.
Linedefs mode: event arrows were not redrawn after updating vertex insert indicator.
Configurations: some titles were too long to be displayed properly in the interface.

Texture Browser: added "Show -> By selection type" option. When this option is selected, the browser will show textures or flats based on selected/highlighted surfaces.

Numeric textbox: in addition to '++NN' and '--NN', you can use '*NN' and '/NN' to multiply or divide the values.
Configurations, ZDoom_linedefs.cfg: some linedef action arguments were missing a proper type.
Rewritten parts of hints system. Hints are now loaded from "Hints.cfg" file.

Added ThingSlope patch by Boris (adds support for Sector tilt things (9502 and 9503) in Visual mode).

Fixed an update logic error in PairedIntControl.

Hints and Draw Settings panels were not removed during map closing/switching, which resulted in duplicate panels.
The way things were rendered during dragging is now used by default in Things mode.

Visual mode, "Select" action with "With the same texture" modifier: sidedefs are no longer selected when matching part has zero height.

When Draw Rectangle or Draw Ellipse mode is enabled, it's settings are now shown in side panel.
Draw Ellipse mode: increased maximum number of sides to 512.
Changed the way tags are shown in Tag Selector controls. Tags without labels are now shown as a number (not "Tag N"), and tags with label are now shown as "N (label)", not "label (N)".
Tag labels are now shown in Thing, Sector and Linedef info panels.
Once again changed the way things are rendered while dragged.
Rearranged the label in PairedIntControl and PairedFieldControl (it is now behind numeric controls).
Optimized MapSet.GetSectorByCoordinates().

Sectors mode, rectangular selection: you can now hold Alt to select things inside of selected sectors.
Sector highlight is now shown when selecting sectors in Tag Explorer.

Visual mode: using "Select" action with "select with the same texture" modifier in some cases selected middle texture when it shouldn't have.

Error checks: added "Check unused textures" option.
Replaced MissingTexture3D and UnknownTexture3D.
Sectors mode: restored "Lower/Raise Floor/Ceiling by 8 mp" actions.
Visual mode: in some cases sidedefs were rendered as selected when they were not.
Existing linedefs were not split while drawing new lines in some cases.
Texture and height overrides were not applied correctly in some cases.
Preferences form: "Ctrl+Alt+ScrollUp" and "Ctrl+Alt+ScrollDown" dropdown items were setting the shortcut to "Ctrl+Shift+ScrollUp" and "Ctrl+Shift+ScrollDown".

Nodes Viewer: ZNODES were properly shown only on freshly opened maps.

Sector highlight is now shown in many more places.
Edit area auto-focusing is now disabled when any form is open.

Fixed a crash when enabling Nodes Viewer mode in an unsaved map.

Fixed: texture size label's visibility was inconsistent in ImageSelectorControls.

Program version, including revision number, is now updated and handled properly.

We should copy middle texture to upper/lower parts regardless of whether they are actually required when AutoClearSidedefTextures is disabled, I suppose (aaaaand there was a logic error there).

Reverted "Delete Item" action to the way it worked in DB2.
Added "Dissolve Item" action, which works the way "Delete Item" worked in previous revisions of GZDB.
Added "Auto Clear Sidedef Textures" action, "Edit" menu and toolbar button, which toggle automatic removal of sidedef textures when floor or ceiling height is changed or when geometry is drawn, copied or pasted.
Draw Settings panel: upper/lower texture overrides can now be used.
Draw Settings panel: added 2 sets of buttons, which allow to quickly set or clear textures in current selection.
Things are now rendered behind AND on top of the grid/linedefs/vertices when they are dragged.
Redesigned hints system. They are now shown in a side panel.
Edit area auto-focusing is now disabled when script editor is open.
Texture Browser form: no texture group was selected when opening the form in some cases.
Fixed several strange/misleading text messages.

Action descriptions: added "disregardalt" parameter.

Visual mode: added "Match Brightness" action.
DECORATE parser: actor names, which contain "+" and "-" symbols, are now parsed correctly.
Fixed an error in lump ranges creation logic.
Cosmetic fixes in various warning/error messages.
Updated documentation.

Added hints for Drag Geometry modes.

Classic modes: bilinear filtering was used in full brightness mode regardless of preferences setting.
Classic modes: "Paint Selection" and "Toggle Full Brightness" actions can now be used from any plugin.
Fixed possible crash in DECORATE actor color override logic.