Ideally we would put the bindings dirrectly in the builtins.60
But this is unfortunately not possible because the "StyleMetrics" is not
yet available there and that would be a circular dependencies.
This allows implementing another sliding puzzle theme feature, the
boldness of the tile number text in the simple theme when the tile is in
the correct position. For the other themes the weight is constant and
it's bold.
More cleanup to follow later: unify the font handle and FontCacheKey
handling, improve the API to allow for the use of symbolic constants
(normal, bold, etc.) instead of the CSS number values.
