Improved support for Dark Mod standalone

- Added new .game file for the Dark Mod, which does not use a "/base/"
  subdirectory under the main engine path. This should remove the need for
  setting the root directory as the engine path.
- Modified Linux code to only look in a home-directory engine path if the
  "prefix" game file property is non-empty. Dark Mod maps are now saved into
  the main installation maps directory on Linux.
Swap EntityClassChooser buttons to match order in 1.8.x
Resolve #3975: Drag-manipulated func_statics can go off-screen with only origin left for drawing. This is due to a duplicate application of the current transformation matrix in Patch::freezeTransform(). I tested a few things, but I can't guarantee that this doesn't break some other code.
Fix #3976: DR shouldn't catch accelerators when a blocking dialog window is in focus.
Fix #3977: Sort algorithm not working in prefab selector due to use of wxDataViewIconText. Don't use CompareTo either.
Replace wxString::CompareTo with wxString::CmpNoCase.
Merge remote-tracking branch 'origin/master' into mousetool