[bombermaze-dev] theme preview

[Sydney Andrew Tang]

I've finished the code to implement the Theme Preview display in the
preferences dialog. It is no longer a black rectangle. The actual format
of the image is subject to change, of course.

I also cleaned up a painting/erasing bug that could happen when switching
themes in the middle of a match. This bug was due to an imcomplete
rollover to the new theme backend.

Barring any bug fixes, these will be the last code changes that I make for
the 0.6.7 release. I believe that the latest code contains one new string
for the translators to translate.