For the sake of completeness, added a GUI for the hidden "Open as Layer"

2004-09-21  Michael Natterer  <mitch@gimp.org>

	For the sake of completeness, added a GUI for the hidden
	"Open as Layer" feature:

	* app/actions/file-actions.c
	* app/actions/file-commands.[ch]: added "file-open-as-layer"
	action and callback. Abuse the "gimage" field of GimpFileDialog to
	indicate layer opening (it's otherwise unused for file-open).

	* app/dialogs/file-open-dialog.c: if dialog->gimage is non-NULL,
	open the selected files as layers for that image.

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

	* menus/image-menu.xml.in: added it to the menu.