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>
Fix regression introduced by b9d6b628a98fc17f5e92a344450b6336f1a2b2ce

When calculated the palette height based in the size of children,
forgoten add the border size. That moved the palette in the device icons
4 pixels to the bottom, then the gap was miscalcuated and the border
button border was not draw.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>