Move all editor-specific attributes to a new editorclass

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.

desktop_version/src/Entity.cpp

@@ -1257,7 +1257,7 @@ void entityclass::createentity(int xp, int yp, int t, int meta1, int meta2, int
 
 #if !defined(NO_CUSTOM_LEVELS)
     // Special case for gray Warp Zone tileset!
-    const edlevelclass* const room = ed.getroomprop(game.roomx - 100, game.roomy - 100);
+    const edlevelclass* const room = cl.getroomprop(game.roomx - 100, game.roomy - 100);
     bool custom_gray = room->tileset == 3 && room->tilecol == 6;
 #else
     bool custom_gray = false;