Auto-scale fonts in full-screen mode

[Mikolaj]

Replace allFontsScale in config file with an argument to the NotFullscreen constructor only. Deduce the scale otherwise, that is, in fullscreen mode.

The full screen resolution can probably be queried using some SDL function. There may be a single subtlety, namely some auto-scaling in high or low DPI mode or Retina displays in OSX. If there is a choice, the game should prefer to scale on its own to the full resolution instead of being auto-scaled by OS.