Bug 344684 - Greyscale (no alpha) clipboard brushes are of the wrong type

Add a "mask-only" property to GimpBrushClipboard. When TRUE, only
create a brush mask (not a pixmap brush with mask and image).

Keep two clipboard brushes around: one classic "Clipboard Image" one
to be used as "stamp", and one new "Clipboard Mask" one that turns the
clipboard into a brush mask.