player: drop StatusStarting

It's useless as implemented right now.
player: use property-change instead of pause, unpause and metadata-update
bus: rename List -> Tracks and make it a property

It now returns only the file list (no count and no current position). The
playlist count can be extracted from the list length, and to determine the
current position, the TrackPath property can be used.
player: separate creation and starting of mpv

This prevents the player from handling events before the bus is initialized.