audio: Limit post tee queue size

Not sure how small we can safely make this, but basically with the volume
element in front of the tee we "need" this as small as possible so the volume
changes fell snappy. Alternative would be one volume element per tee branch.

mopidy/audio/actor.py

@@ -164,7 +164,9 @@ class _Outputs(gst.Bin):
 
     def _add(self, element):
         # All tee branches need a queue in front of them.
+        # But keep the queue short so the volume change isn't to slow:
         queue = gst.element_factory_make('queue')
+        queue.set_property('max-size-buffers', 5)
         self.add(element)
         self.add(queue)
         queue.link(element)