Preferences redesign

[amka]

While we're at it, the Language list is quite ugly, and the scrollbar is in the wrong spot:

I'd remove the padding from the main box, putting it on the list instead, set selection-mode: none on it, make the download buttons the activatable widgets and used AdwClampScrollable instead of AdwClamp. In addition, .flat isn't really supposed to be used with scrolled views so it intersects the list nastily.

Originally posted by @jannuary in #50 (comment)

• Use Adw.PreferencesWindow instead of Gtk.Window
• Split language list into Available and Installed
• Move general settings into General Adw.PreferencesPage
• Add progress indictor into language row