2004-05-04  Michael Natterer  <mitch@gimp.org>

	* app/core/gimpimage.c (gimp_image_undo_freeze,thaw): emit the
	"undo-freeze" and "undo-thaw" signals only on the first freeze and
	last thaw, not on any of them.

	* app/widgets/gimphelp-ids.h: added GIMP_HELP_EDIT_UNDO_CLEAR.

	* app/widgets/gimpundoeditor.[ch]: added a "Clear Undo History"
	button. Fixes bug #136300.

	Also don't attach to the image's undo stack if the image's undo is
	disabled and set the buttons' sensitivity accordingly. Should fix
	all kinds of unpredictable undo history brokenness.