Bug 732072: Image windows created by script-fu plug-ins...

are opened behind the previously focused Image Window

libgimp/gimpui.c bypasses gtk in setting the transient_for
property and so it is not possible to call
gtk_set_transient_for (dialog, NULL), but it is necessary
to go directly to the gdk level.

(cherry picked from commit 51d3498cd4c63edc4233520ca737a3d0e1a1b185)