Migrate Gtk.Stack to Adw.NavigationView
Closes #46 (closed)
A lot of changes were needed, for instance in the way the spinner works. The DnD overlay was also changed to Amberol's implementation.
This new layout is not supposed to be used as it is now, but should rather provide a building block for writing a queue system. Therefore, some things in this MR may still be a bit funky.