* app/undo.c: Fixed evil<tm> double casting of enum to glong to
	gpointer for layer_mod undo by using a struct instead of an array
	of gpointer (!!!) for storing layer_mod undo data.  Also moved
	layer offset undo information out of the undo tile manager.
	* app/image_map.c: Moved layer offset undo information out of the
	undo tile manager.

	Above changes may break image_map and layer_mod undos, please test
	as I haven't. :)