Merge branch '287-sort-by-tracknumber' into 'master'

UI: Sort songs by track number in Browse mode

See merge request sublime-music/sublime-music!69

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) -> Tuple[Optional[int], Optional[int]]:
+                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 = [
                 [
                     (