Fix default fonts in RH8.0. We use the style hint after all else fails

Fix default fonts in RH8.0. We use the style hint after all else fails
in order to find a reasonable font.

Note that StyleSerif does not work. I blame RedHat 8.0 personally,
but cannot verify yet.

Note also that in a sane world, we could just use font.setFamily(blah);
QFontInfo(font).family() and set to that. But Qt is obviously not sane, so
we stay with the weird setComboxFont() code we have :(

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@5869 a592a061-630c-0410-9148-cb99ea01b6c8