PR adjustments

Hi there, 

thanks a ton for your Pull Request!

Here are some adjustments for the PR. Really just the last 3 are important, the rest seems to be stuff that happened on shoes4 master while you worked :-)

The biggest two things I did were:

- make Shoes::Font a singleton, as we never really need a Shoes::Font instance
- populate Shoes::FONTS in the beginning and then gradually add to that... there are some outstanding issues which I will comment on in the original PR

Cheers,
Tobi