This is a pretty hefty commit! But essentially, I made a new editorclass
object, and moved all functions and variables that only get used in the
in-game level editor to that class. This cleanly demarcates which things
are in the editor and which things are just general custom level stuff.
Then I fixed up all the callers. I also fixed up some NO_CUSTOM_LEVELS
and NO_EDITOR ifdefs, too, in several places.
As far as I can tell, this function has never been implemented, and only
existed in this header file. FILESYSTEM_getLevelDirFileNames() already
exists (well, used to exist; it's been changed and renamed to
FILESYSTEM_enumerateLevelDirFileNames()), so I'm removing this now.
This accompanies the editor.cpp -> CustomLevels.cpp change; I'll be
splitting out the editor functions in the next commit. The name of the
include guard has been changed as well, but not anything else.
