Add diagonal symmetry to board setup pane.

A simple feature, turned on by default, which makes setting up
the board even faster.
Add color theme feature.

This allows you to choose between preset themes stored in .ini files
in the layout.  While I was at it, I changed other things in the
Settings panel to auto-populate from the layout rather than have
hard-coded versions.  So there's no longer a need to recompile
Quackle to add new languages or alphabets to get it to show up in
the popup menu.