Save and restore the playing state when the app restarts

[flodavid]

Add new settings where current track and queue information (type and id of playlist) are stored : last-played, queue-type, queue-id, track-progression.
Those data are stored when the track or queue changes, and when the current playing track progression changes (every two seconds).
When the application starts, the saved data are read and the current queue, track played and progression are restored. Next track information is not stored, so when "shuffle" is used, the next tracks of the playlist are shuffled again.

• I did not implement restoring from Radio
• When a track is not found, we now read the next one instead of stopping playback.
• Also fixes the database method get_track_by_id to retrieve the Artist name from the Track, not the Artist of the Album. It could lead to errors.