playlist not persisted when closing podqast

Reported by Danton

1. Start a manual refresh in the "library".
2. Check that new episodes were added to the playlist.
3. Close podqast.
4. Restart podqast -> episodes added during the last refresh are now missing from the the playlist, although the files are still downloaded. Mitigation at the moment:

Following the refresh, start playing the first episode for a moment before closing podqast.

Suspected reason: the playlist is pickled, not in sqlite. After refresh the playlist should be persisted to the filesystem