UI: Sort songs by track number in Browse mode

This commit may address issue #287. It simply takes the list of songs
provided by the server in Browse mode and sorts it by disc and track
number. Tests seem to have passed except for (unrelated?) mypy issues.

sublime_music/ui/browse.py

@@ -377,6 +377,14 @@ class MusicDirectoryList(Gtk.Box):
                 self.drilldown_directories_store, new_directories_store
             )
 
+            def song_sort_key(song: API.Song):
+                return (
+                    song.disc_number if hasattr(song, "disc_number") else 0,
+                    song.track if hasattr(song, "track") else 0,
+                )
+
+            songs.sort(key=song_sort_key)
+
             new_songs_store = [
                 [
                     (