Also, it's ugly to blank out pieces of the applet window in black.

Use the puzzle background colour, like the GTK front end does.

(I know that renders the effect of the previous commit invisible,
but it's the principle of the thing! :-)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9023 cda61777-01e9-0310-a592-d414129be87e