player: properly emit signals on stop
dbus-service: turn some methods into properties

Leave TrackPosition as a method for now since the crappy glib dbus wrapper
doesn't support org.freedesktop.DBus.Property.EmitsChangedSignal.
dbus-service: don't emit signals anymore

Both StatusChanged and TrackChanged are covered by the signal emitted when a
propery changes (org.freedesktop.DBus.Properties.PropertiesChanged). The other
signal (OptionChanged) was unused.