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.
We said we were going to eliminate this antipattern.

see https://github.com/mixxxdj/mixxx/commit/87ef52