Image exporter: added options to use sector brightness for the export, create brightmaps (based on sector brightness), and split the image into 64x64 tiles

Sectors Mode: the sectors things are in are only determined if necessary, which should increase performance when switching into Sectors Mode in very big maps

3D Floor Mode: fixed a crash when duplicating 3D floor geometry and there was not enough space in the control sector area

Fixed a bug introduced by addressing #427 that stopped the script editor to show up in vanilla Hexen. Fixes #429.

Fixed a problem where translucent 3D floors were drawn solid. The cause was a rounding error because of the transition from float to double.

The script editor is now only shown if the current game configuration supports scripts. Resolves #427.

Fixed a crash in the DIALOG/SCRIPTS editor when selecting any other than the first two options in the "look in" combo box in the replace tab. Fixes #425.

Draw Ellipse Mode: fixed a rounding error that could result in wrong shapes under certain conditions

Edit Selection Mode, Flat Align Mode: fixed a bug where snapping occurred to wrong positions on lines when the grid was transformed

Draw Rectangle Mode: fixed a problem where the guidelines were not displayed correctly under certain conditions

Things Mode: performing the smart grid transform action will now set the grid origin to the selected or highlighted thing
Sectors Mode: performing the smart grid transform action will now reset the grid transform

Vertices Mode: the preview vertex is now correctly shown on intersections between the closest line and the transformed grid

Added some error handling to duplicating 3D floors

Changed how performing undo interacts with volatile modes (it will now actually perform the undo instead of just canceling the volatile mode)

- Added preliminary code to remove new 3D floor control sectors when pasting was canceled after duplicating 3D floors

Added functionality to duplicate 3D floor control sectors of selected sectors and paste selected sectors with the new control sectors assigned

3D Floor Mode: fixed a bug with the "Detach all" and "Split all" that would create (and immediately delete) redundant control sectors

Draw Grid Mode: added option to to draw the slices relative to the origin of the drawing (so that you don't have to change the slice interpolation all the time)

Added the "nocount" UDMF thing flag (as "No completion count") that was added in GZDoom 4.4.0 to the ZDoom config. Fixes #422.

Added comments to code