smoothscale: Do not interact with GstPlayer

See the discussion from !285 (merged)

The state property bothers me. I had to make two changes which I do not completely understand:

• remove message::state-changed listener in GstPlayer`
• add a self.state=self.state in seek method to notify of the state change

Without the state-changed, the notify::state listener get way too many notifications and PlayerToolbar becomes unresponsive (the mpris state changed operation is probably expensive).

Once these changes applied, we will be able to update !285 (merged).