PlaylistHeader: Add a new header to the playlist view

This merge request brings an addition to the playlist view with:

• The addition of a header displaying a bigger artwork cover with buttons more accessible to start the playback regularly and shuffled.
• The EditController now also displays the playlist header when the collection is being reordered by the user.

This merge request also brings an adjustment to the playlist sorting criteria:

• The sortModel is properly instantiated in the playlistModel instead of being handled by the collectionModel.

Screenshots

Closes #1868 (closed)