Improve font selection

[spodskubka]

Users should be able to choose every fixed-width font that is available on their system, not only the limited list that is selectable now.

If that is not possible there should be some advanced fonts that users have requested bundled with the app (if licenses permit it):

• https://dejavu-fonts.github.io/ or https://github.com/dejavu-fonts/dejavu-fonts
• https://github.com/powerline/fonts

[DRSDavidSoft]

In addition to those, I'd suggest Source Code Pro which is a very nice font:

• https://github.com/adobe-fonts/source-code-pro

Also, based on that there is Hasklig which also would be nice.