Visual Mode: fixed a problem where things directly touching the top of 3D floors were rendered with the incorrect brightness. Fixes #940

Visual Mode: changes to Eternity Engine's upper/lower texture skewing definitions are now shown in Visual Mode

Visual Mode: fixed a crash when toggling visual vertex slope picking and the map contains double-sided linedefs without a front sidedef. Fixes #928

- fixed an issue where undo snapshots were not created correctly when editing a sidedef's custom UDMF fields. Fixes #920
- fixed an issue where closing the back sidedef's custom UDMF field editing dialog would close the linedef dialog

- fixed a rare condition where skewed mid textures were not clipped correctly
- fixed a problem where translucent mid textures were not skewed correctly
- fixed a problem where sprites were skewed unintentionally

Draw Grid Mode: fixed a problem where "continuous drawing" would redraw all previously drawn shapes each time a drawing was finished. Fixes #912

UDBScript: fixed an issue where the Thing's class getSector method did not return null when the thing was not in a sector. Fixes #897

Added action to change the index of linedefs, things, sectors, and vertices. Can be accessed through a shortcut (not bound by default) or the menu of the respective mode (#903)

The calls to native windows functions wouldn't be valid on any unix os
so expand the check.

The check for Windows XP is removed as that platform is not supported
anymore.

When saving a wad for the second time some code would run that tried to
use native windows function and caused a crash.

- A new "Show Textures" option up on the top bar, which draws any sector not flagged to be hidden on the textured automap
- Holding Shift will highlight sectors instead of lines
- Left-clicking on a highlighted sector will toggle the "Not shown on textured automap" flag
- Textures will be shown when Shift is held down even if the "Show Textures" toggle is off

EDGE Classic Game Configuration: fixed a problem with lighting effects in the generalized sector options. Fixes #875