Goal

Populate library with one new item per directory exposed by Mopidy-File (with folder icon).
Clicking on a directory item displays the library browser with one item for sub-directory or for track.

Populating library with resources exposed through Mopidy-File extension should be configurable through preferences dialog.

See #109 for original request.

Details

• Introduce new model type for "Directories"
  cf. https://docs.mopidy.com/en/latest/api/models/#data-model-relations
• Extend the Gtk.IconView contained in current AlbumsWindow to support display of albums, directories, and tracks.
• Rename Python module albumswindow, class AlbumsWindow to librarybrowser, LibraryBrowser, etc.
• Add a MopidyFileBackend class inheriting MopidyBackend and populating model from resources exposed through Mopidy-File extension.