gtk: Introduce private GtkFontFilter helper
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 returnGTK_FILTER_MATCH_ALL
when nothing is selected yet (which is not possible with a `GtkCustomFilter'). It's not much, but it's still nice