Variable fonts support in Qt

More and more fonts are coming as so-called "variable fonts" where multiple styles are stored in the same file.

For system-installed fonts, this should work (something to verify as well), since the system does the resolution for us, and we never deal with the font file itself. But for application fonts, we do not resolve the font styles correctly and may assume in several places that a single font file always designates a single font.

Most variable fonts I've seen come with the static fonts as a backup solution, so that's probably a valid work-around for now, but as this is becoming more and more common, we are going to see more and more reports and frustration about it.