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/RenderFixed.cpp

@@ -1,6 +1,7 @@
 #include "CustomLevels.h"
 #include "Game.h"
 #include "Graphics.h"
+#include "Editor.h"
 #include "Entity.h"
 #include "Enums.h"
 #include "Map.h"
@@ -118,7 +119,7 @@ void gamerenderfixed(void)
         map.glitchname = map.getglitchname(game.roomx, game.roomy);
     }
 
-#ifndef NO_CUSTOM_LEVELS
+#if !defined(NO_CUSTOM_LEVELS) && !defined(NO_EDITOR)
     ed.oldreturneditoralpha = ed.returneditoralpha;
     if (map.custommode && !map.custommodeforreal && ed.returneditoralpha > 0)
     {