2002-08-20  Michael Natterer  <mitch@gimp.org>

	Make sure the selection (gimpimage-mask.c) functionality is really
	built *on top* of the GimpChannel functionality:

	* app/undo.[ch]: renamed undo_push_image_mask() to
	undo_push_mask() and generalized it's API to take a GimpChannel
	param so undos can be pushed for channels which are not the
	image's selection. Simplified the API and added code which copies
	the region of interest instead of leaving this to callers.

	* app/undo_types.h: s/IMAGE_MASK_UNDO/MASK_UNDO/

	* app/undo_history.c: changed accordingly.

	* app/core/gimpchannel.[ch]: don't #include "gimpimage-mask.h".
	Changed gimp_channel_push_undo() to really push a channel undo,
	not a selection undo. Added "gboolean push_undo" params to all
	functions which are called from gimpimage-mask.c. Various cleanups
	and optimizations. Added /*< proxy-foo >*/ stuff to the header so
	we export just the struct itself to libgimpproxy...