make fonts easier to load; add default font function

Quick font fixes, primarily to make it easier to support non-Latin languages.

1) Add a `set_default_font()` function to change the global default.
2) Make it easier to load fonts. Kivy includes DroidSans.ttf and DejaVuSans.ttf in the same folder (data/fonts). The default `font_name` is 'DroidSans', but setting the `font_name` to 'DejaVuSans' didn't work when it seems like it should.

Just give feedback for now, do not merge yet. This probably won't apply cleanly with #2411 due to adjacent hunks, and I will still need to add some docstrings.