2003-03-17  Michael Natterer  <mitch@gimp.org>

	Made drawable/layer properties (visibility, opacity etc.)
	undoable (fixes bug #73893).

	* app/core/core-enums.[ch]: added undo types/groups for
	visibility, mode, opacity, linked and preserve_trans.

	* app/core/Makefile.am
	* app/core/core-types.h
	* app/core/gimpitemundo.[ch]: new GimpUndo subclass which holds a
	ref'ed GimpItem pointer so (1) this doesn't need to be done by all
	undo steps related to an item and (2) the item the undo step is
	for can be determined from outside the undo system.

	* app/core/gimpimage-undo.[ch]: added gimp_image_undo_push_item()
	which returns a new GimpItemUndo.

	* app/core/gimpimage-undo-push.[ch]: use it for all item related
	undo steps. Removed lots of GimpItem, GimpLayer, GimpDrawable
	and GimpVectors pointers from the private undo structs. Added
	undo push functions for the new undo types added above.

	* app/core/gimpdrawable.[ch] (gimp_dra...