Visually distinguish charset headings in GTK3 unifontsel.

Visually distinguish charset headings in GTK3 unifontsel.

When displaying a server-side font, the unified font selector's
font-style list box contains some lines which are character-set
headings, and others which are actually selectable font styles. We tag
the former with the "sensitive"=FALSE attribute, to prevent them from
responding to clicks. In GTK2, this also made them visually distinct
from the normal lines, by greying them out; in GTK3 it makes no visual
difference.

The simplest solution is to bold those lines, hinting that they're
sort of section headings. That looks OK in GTK2 as well, so I've done
it unconditionally.