Draw a black background in the buttons when the palette is up, as
commit 01a06943 did with the ToolButton. As the comment for that
commit states, we can change the prelight background color in the
theme, but not when the mouse moves over the Palette.
Also add testcase to test the three toolbuttons.
Signed-off-by: Manuel Quiñones <manuq@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>
