Add library font preference.

* Add font selector to the preferences.
* Move row height preference from Interface to Library.
* Add controls for changing the font size:
  - [Library],font_size_increment
  - [Library],font_size_decrement
  - [Library],font_size_knob

If the row height is smaller than the font-size the larger of the two is
used.
> ctrl+mousewheel?

Please no, also https://bugs.launchpad.net/mixxx/+bug/661717/comments/10