get_path_to_playlist: use correct formatting
Add songs to playlist from albums view
grilo: Add 'remaining' param to populate_items

Also, change uses of this method in view.py and widgets.py
window: Make fetching of selected items asynchronous

Fetching the selected items should be asynchronous, so we could execute
queries to get the songs from a certain album or artist.
view: Complete add to playlist in Albums view
view: Implement add to playlist in Artists view