When gimp_plug_in_close()ing a plug-in (such as when cancel is
clicked), make sure we don't leak the proc_frames of all temporary
procedures the plug-in is currently running. By properly disposing the
GimpPlugInProcFrame, the progress window and its cancel button in
above bug get destroed on first click, and the user has no chance of
trying to kill an already finalized plug-in again.
(cherry picked from commit eac70cc513)