#549 More default font sizes

[Makarius]

The following modest patch provides a few more default font sizes for the font selector. This is e.g. relevant for current 4K displays on Linux and probably also on Windows (on Mac OS X Retina works differently). Note that my current default size is 30, which is not really big on Dell UP3214Q.

There is also a more generous guess at the preferred size, although that could be done more dynamically according to the actual size of the selection.

[Alan Ezust]

Committed 23904.