2004-03-15  Michael Natterer  <mitch@gimp.org>

	Prepare the undo system for proper text layer undo:

	* app/core/core-enums.[ch] (enum GimpUndoType): replaced
	GIMP_UNDO_IMAGE and GIMP_UNDO_IMAGE_MOD by GIMP_UNDO_DRAWABLE.

	* app/core/gimpimage-undo-push.[ch]: ditto: replaced
	gimp_image_undo_push_image() and gimp_image_undo_push_image_mod()
	by gimp_image_undo_push_drawable() which *always* expects to get a
	TileManager passed. Also added g_return_if_fail()s to check if the
	passed in tile manager follows the semantics of the "sparse"
	boolean.

	(undo_pop_drawable): removed all code and call the new
	gimp_drawable_swap_pixels() instead (see below).

	* app/core/gimpdrawable.[ch] (gimp_drawable_push_undo): if tiles
	are NULL, create a copy of the area here and always pass tiles to
	gimp_image_undo_push_drawable(). Added lots of g_return_if_fail()
	here too.

	Added new vitrual function GimpDrawable::swap_pixels() which
	does what undo_pop_drawable() did.

	* app/core/gimpchannel.c: implement swap_pixels() and invalidate
	the channel's bounds and boundary.