This is needed for the color widget to have the correct size at
initial show, and to keep this size after selecting a different
palette.

The parent FloatingWindow will assume that the border width stays
outside the space that was allocated to the DockingWindow (see
VclContainer::setLayoutPosSize), and yet DockingWindow tries to
handle the border width as part of its allocated space. One option
could be to let FloatingWindow handle this alone, but this won't
work for other possible parents. The current solution is to load
and store the border width in a way that can be used internally,
but not visible to the outside world via get_border_width().

Change-Id: Id51152cf64eef719368e29253eb93e99834489cd