2005-01-13  Michael Natterer  <mitch@gimp.org>

	Merged from HEAD branch:

	Splitting GimpItem::convert() into two virtual functions was
	apparently buggy over-engineering. Reverted that stuff and
	implement item conversion much simpler:

	* app/core/gimpitem.[ch]: have a single virtual function,
	GimpItem::convert(), which takes a newly duplicated item and the
	destination image.

	(gimp_item_convert): simply call GimpItem::convert() on the result
	of gimp_item_duplicate().

	(gimp_item_real_convert): set the item's new image.

	* app/core/gimplayer.c (gimp_layer_convert)
	* app/vectors/gimpvectors.c (gimp_vectors_convert): changed
	accordingly and chain up after, not before doing our own stuff
	so the old image is still available as item->image for stuff
	like colormap conversion. Fixes bug #163879.