Centering didn't work if the text below the thumbnail was wider than the
thumbnail itself. Fixed.

The ButtonRelease check only knew pan mode, so releasing the blur key changed
to the next image.

I know that there is one in the old SVN repo, but I think a rewrite
(with an automated test script) is due.

The menu background images were not properly free()d

Does not yet work with initial zoom value != 100%