Fonts: Add "slim" and "wide" variants of the System font

These are not included in the build or packages. If you want to try
them out you need to be set up to build DeskTop. Replace the normal
mgtk/fonts/System.latin1 file with one of the variants, and rebuild.

The differences are:

* Slim: Make 'b' and 'd' as skinny as 'p' and 'q'. Once you become
  aware of this inconsistency you can't unsee it.

* Wide: Increase the width of counters in font glyphs like eghnopquvy,
  digits, and a few other characters like ? and V, and their non-ASCII
  cousins to improve legibility.

See discussion in #793