Extract the "user filter" code from the `GtkFontChooserWidget`, for a
couple of reasons:

* If we want to expand the filter in the future (e.g. to filter on
  variable fonts, or check for multiple languages), we have a nice place
  to put this.
* It simplifies the font chooser widget a tiny bit, as it's a pretty big
  file which can be hard to follow.
* With a custom `GtkFilter` subclass, we can actually avoid doing a bit
  of work when initially showing the widget, as we can return
  `GTK_FILTER_MATCH_ALL` when nothing is selected yet (which is not
  possible with a `GtkCustomFilter'). It's not much, but it's still nice