'Called after the playback state have changed.
Will be called for both immediate and async state changes in GStreamer.
Target state is used to when we should be in the target state, but
temporarily need to switch to an other state. A typical example of this
is buffering. When this happens an event with
`old=PLAYING, new=PAUSED, target=PLAYING` will be emitted. Once we have
caught up a `old=PAUSED, new=PLAYING, target=None` event will be
be generated.
Regular state changes will not have target state set as they are final
states which should be stable.
*MAY* be implemented by actor.
:param old_state: the state before the change
:type old_state: string from :class:`mopidy.core.PlaybackState` field
:param new_state: the state after the change
:type new_state: A :class:`mopidy.core.PlaybackState` field
:type new_state: string from :class:`mopidy.core.PlaybackState` field
:param target_state: the intended state
:type target_state: string from :class:`mopidy.core.PlaybackState`
field or :class:`None` if this is a final state.'

def state_changed(self, old_state, new_state, target_state):

    pass
