Fixed several crashes when trying to load not existing images and models from folder/pk3/pk7 resources.

Visual mode: fixed a crash while changing 3d floor texture offsets when visual sector was not created for 3d floor's control sector.
Visual mode: Info/Warning/Error status messages were not displayed properly when nothing was selected.
Sector Edit form, UDMF: Reset and Link buttons were overlapped in Floor/Ceiling scale controls.

Textures now load up to 2x faster when "mix textures and flats" flag is set in game configuration.
TEXTUREx/TEXTURES: texture will now be created if at least one of it's patches is loaded.
Visual mode: fixed a crash when "Slope floor to here" (9500) or "Slope ceiling to here" (9501) things were not inside sector.
Fixed: flats were not loaded form wads inside Directory and PK3/PK7 resources.
Sector Info Panel, Linedef Info Panel: texture size was shown for unknown textures.

Fixed some errors in non-UDMF <-> UDMF map conversion.
Updated ISS setup script (not tested).
Removed some unused variables.

Texture scale controls now have "Link" button. 
Thing, Sector (UDMF) and Linedef edit forms now work in realtime mode.
Some more fixes in Edit form realtime update stuff.
Removed some unused variables and functions.

Sector Edit form, Vertex Edit form: fixed a crash when updating sector/vertex properties in Visual mode.
Reorganized game configurations.

Sector Edit form, UDMF: sectors were not updated properly after closing the form because of changes in Doom/Hexen Sector Edit form.

Status text now displays info about current selection.
Removed StairSectorBuilder plugin. The version form doombuilder.com works fine with current version of GZDB.

Tag Statistics form: you can now double click on Sectors, Linedefs or Things cell to select them, and right click to open their properties.
Texture size labels were displayed incorrectly in some cases.
Rewritten VerticesMode.DeleteItem() once again...
Vertex Edit form now works in realtime mode.
Vertex Edit form: ceiling and floor vertex offsets can now be cleared.
Added StairSectorBuilder plugin (I suppose some external plugins will stop working in GZDB because I've changed ButtonStep to float in ButtonsNumericTextbox a couple revisions ago...).
Preferences form: action description is now scrollable.
Changed background color of Sector Edit form.
Vertex' ZCeiling and ZFloor properties are now managed internally.

Rewrote VerticesMode.DeleteItem function. It was still not working properly after previous commit (proper vertex deletion can become surprisingly complicated in some cases)...

Visual mode: you can hold Shift while dragging a texture to lock movement to horizontal axis, and Ctrl to lock it to vertical axis.
Color Picker: fixed a crash when entered float values were not in 0.0 - 1.0 range.

Visual mode, UDMF: OffsetX and OffsetY were not properly taken into account in "Fit Texture Width/Height" actions.
UDMF: different Sector Edit Form is now used in UDMF map format.
Some fixes in Linedef edit form.
Added missing icons.

Visual mode, UDMF: added "Scale Texture Up (X)", "Scale Texture Down (X)", "Scale Texture Up (Y)", "Scale Texture Down (Y)" actions. Default keys are Num6, Num4, Num8, Num5.
Visual mode, UDMF: renamed "Rotate Thing Clockwise" and "Rotate Thing Counterclockwise" actions to "Rotate Clockwise" and "Rotate Counterclockwise". These actions can now be used to change rotation of floor/ceiling textures.
Visual mode, UDMF: "Reset Texture Offsets" action now also resets sidedef's scale and floor/ceiling's scale and rotation.
Visual mode, UDMF: control line's OffsetX and OffsetY were not taken into account when calculating texture offsets of 3d floors' sides.
Visual mode, UDMF: fixed a ton of bugs in Auto align functions.
Visual mode, UDMF: when using "Move Texture Left/Right/Up/Down by 1" actions texture offsets were not updated properly when texture's scale was < 1.0.
Visual mode, UDMF: OffsetX and OffsetY were not taken into account in "Fit Texture Width/Height" actions.
Dockers Panel: added Pin/Unpin button, which acts the same as "Preferences -> Interface -> Side panels -> Auto hide" checkbox.
Texture size labels can now be disabled by unchecking "Preferences -> Interface -> Show texture and flat sizes in browsers" checkbox.
Texture size labels now are not shown for unknown textures.
Most of texture size labels had incorrect bg color.
ZDoom_linedefs.cfg: action specials 223 and 224 had incorrect Arg0.

Visual mode: fixed a crash after disabling GZDoom rendering effects (Tab key) when visual sector was not created for sector with 3d floor's control linedef.

Fixed several bugs in Engine selector dialog ("Game configurations -> Testing").

Map Analysis mode: only a single undo is now created when multiple similar errors are fixed using "Apply to all errors of this type" checkbox, which greatly reduces errors fixing time.

Removed some unused "using" directives.

Linedefs mode: added "Align Floor Texture to Front Side", "Align Floor Texture to Back Side", "Align Ceiling Texture to Front Side" and "Align Ceiling Texture to Back Side" actions (available in Linedefs -> Align Textures menu).

Visual mode, UDMF: "Auto-align textures" actions now work for floors and ceilings. Textures are aligned to a linedef of highlighted floor/ceiling, which is closest to 3d-cursor position.

Linedef colors were not updated properly when a map format was changed and were causing an exception in some cases.
Vertices mode: linedef in split linedefs range was redreawn incorrectly in some cases.
Updated "Sector 3D Floor" (160) action: set default Type to Solid (1), added "Use Arg5 as Line ID" flag to Types, changed arg5 name to "Line ID / Hi-Tag".
Added "GetUserCVar", "SetCVar" and "SetUserCVar" functions to ACS configuration.
Set width and height of several actors to 0 in game configutrations (non-zero sizes were added as a workaround when GZDB was not displaying actors with 0 width and height in Visual mode).
Added several sprites to game configurations.

Classic Modes: added 3D floor indication (color can be changed in Preferences -> Appearance -> 3D Floors).
Sector_Set3dFloor action (160): added hi-tag/line ID check.

Visual Mode, UDMF, Visual Vertices: heights of vertices without z-offset was incorrect in some cases.
Visual Mode, UDMF, Visual Vertices: vertices without z-offset are now rendered using Vertex color (Preferences -> Appearance -> Vertices).

Visual Mode, UDMF: added "Fit Texture", "Fit Texture's Width" and "Fit Texture's Height" Actions.cfg
Visual Mode, UDMF, Visual Vertices: you can now press "Delete" key to clear z-offsets of selected vertices.
Visual Mode, UDMF, Visual Vertices: the logic, which decides whether z-offset should be kept or removed, was not working properly.
Visual Mode, UDMF: Auto-align Textures (Y) action was not working properly when targeted texture was lower wall without "Lower Unpegged" flag.

Visual Mode, UDMF: "Auto Align Textures" actions now work on slopes.
Visual Mode, UDMF: Floor/ceiling texture rotation is now taken into account when moving texture offsets with arrow keys.
Visual Mode, UDMF: Fixed a bug in floor/ceiling texture dragging by mouse.

UDMF: Reset texture offsets action now works on floors and ceilings.
Vertices mode: when a linedef is within Split Linedefs range, it's now highlighted using Info line color.

Fixed: Incorrect articles were sometimes used in Draw Geometry/Curve/Rectangle/Ellipse info messages. 

Classic modes, UDMF: floor/ceiling texture offsets can now be locked in place while sector is dragged.
Fixed another bug in Tools.autoAlignTexturesOnSidesUDMF()
Replaced Math.PI-related equations with stuff from Angle2D class.

Rectangular Selection: removed direction-based additive/subtractive selection. You can now hold Ctrl to enable subtractive selection, hold Ctrl-Shift to intersect with existing selection.
Point Thing at Cursor action: action now works when used from menu, changed default shortcut to Shift-L. You can now hold Ctrl to point Things away from cursor.