Finally implemented the suggestion in bug #144854, of
	"strong" undo/redo commands that continue undoing so long
	as they only encounter visibility changes.

	* app/actions/edit-actions.c
	* app/actions/edit-commands.c
	* app/actions/edit-commands.h: added "strong undo"
	and "strong redo" commands/actions.

	* app/core/gimpimage-undo.[ch]: added functions
	gimp_image_strong_undo() and gimp_image_strong_redo().

	* app/core/gimpundo.[ch]: added utility function
	gimp_undo_is_weak().

	* app/widgets/gimphelp-ids.h:added id's.

	* menus/image-menu.xml.in: added to edit menu,
	bound to C-S-z and C-S-y.

	This will no doubt need tweaking, but I will consider it
	to fix bug #144854.