qt: introduce persistent playlist feature

This feature has been requested multiple times. It simply saves the playlist before closing, and restores when starting.

If the user starts the application already with a media, then the saved playlist is not loaded. If there is no actually a "list" (meaning, multiple files), playlist is not saved.

If possible and when auto-preparse is set, it also restores the current index and the position.

If the user unsets qt-persistent-playlist, the application removes the recorded playlist, index, and position (if there is).

This feature is orthogonal to playlist recovery (!6028 (merged)), which gets engaged when the application closes abruptly (such as crash).

Request review @chub.