Fix drawing of gap on palettes attached to widget - Fixes #4776

In Gtk 3.10, Gtk.Window is drawing a gray border around the palette.
This patch draw a black rectangle defore we draw Gtk.render_frame_gap
to draw the border but with a gap to connect to the attached widget.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>