Color Palette: get back horizontal padding - SL #4325

The PaletteWindow lost the horizontal padding when it started to be
used as replacement of palette menus.  So now each implementation has
to add the padding.

Signed-off-by: Manuel Quiñones <manuq@laptop.org>
Merge pull request #10 from manuq/4325

Color Palette: get back horizontal padding - SL #4325