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.
I've had problems in the past debugging signals that are set up in one object that apply to another object and its slots.  Although it's a little uglier to have a setupConnections() function in the target object, it helps to understand the construction flow.