* app/core/core-enums.[ch]
	* app/core/gimpimage-undo-push.[ch]
	* app/pdb/pdb_glue.h
	* app/actions/layers-commands.c: make layer mask property changes
	undoable, should fix bug #148852.  Note that a little shape-layer
	stuff is in core-enums, will be needed shortly.