terminal: don't even attempt to set a font which won't work
terminal: don't even attempt to set a font which won't work

The passed font has a high chance of not being installed.

So we just test for the font being fixed (or half-fixed), which should
also make sure it is installed.

Also iterate over possible alternative fonts to give more chance of
finding one.

This makes Theme Manager usable again :-)

mmuman/HaikuThemeManager,mmuman/HaikuThemeManager