OBJ Exporter: replaced default folder selection dialog with something more convenient (and more consistent with the rest of the program)

OBJ Exporter: checking the "Solid" checkbox now adds to following flags to the actor: INVULNERABLE, NODAMAGE, SHOOTABLE, NOTAUTOAIMED, NEVERTARGET, DONTTHRUST

OBJ Exporter: improved exporting models for use in GZDoom. It can now export models that can be directly used in GZDoom, and also creates the necessary MODELDEF and ZScript/DECORATE code

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

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

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

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 comments to code

Made sloping work more reliably with irregularly shaped sectors

Improved handling of changing input boxes in slope arching form

Improved functionality