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.

Thing Edit form: several thing types can now be selected at once. In this case, a type randomly chosen from selected ones will be assigned to each selected thing.
Thing Edit form: you can now double-click on thing type to apply the changes and close the form.

Added RandomFloat Universal field type. To use it, change Filed type to "Decimal (random)" and enter a value as 'min max'.

Added RandomInteger Universal field type. To use it, change Filed type to "Integer (random)" and enter a value as 'min max'.

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.

Sector info panel: floor/ceiling brightness should be always shown when "lightabsolute" flag is set.

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.

+ some fixes to previous commit.

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

Linedef edit form: action arguments were set to zero when settings were applied to several lines with different argument values (instead of being kept unchanged).

Linedef edit form: all linedef UDMF properties are now editable via user interface + some fixes in sidedef UDMF controls. 

Fixed a typo in one of Visplane Explorer messages.
Small fixes here and there.