Use Hdy/AdwPreferencesDialog

[bertob]

The preferences are a bit weird-looking, in large part because they don't use listboxes. I'd port as much of it as possible to the standard Hdy/Adw preferences widgets.

For the first section I'd try something like the subsetting preferences in Webfont Kit Generator:

[GustavoPeredo]

I really liked the design, but currently reworking the preferences window would take some time and I prefer spending this time working on (if you have time, your input on the design mockups would be appreciated)