Rename editor.h to CustomLevels.h

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.

desktop_version/src/main.cpp

@@ -1,7 +1,7 @@
 #include <SDL.h>
 
+#include "CustomLevels.h"
 #include "DeferCallbacks.h"
-#include "editor.h"
 #include "Enums.h"
 #include "Entity.h"
 #include "Exit.h"