Cleanup: sync w/ blender2.8 branch

Split screen_draw.c from screen_edit.c (avoid conflicts syncing).

source/blender/editors/include/ED_screen.h

@@ -47,6 +47,7 @@ struct bScreen;
 struct ARegion;
 struct uiBlock;
 struct rcti;
+struct Main;
 
 /* regions */
 void    ED_region_do_listen(struct bScreen *sc, struct ScrArea *sa, struct ARegion *ar, struct wmNotifier *note);