core: Add volume arg to volume_changed() event

It was already called with the argument, and both the MPD and HTTP frontends
handled it/expected it. It was just the default implementation in CoreListener
that lacked the argument.

mopidy/core/listener.py

@@ -132,11 +132,14 @@ class CoreListener(object):
         """
         pass
 
-    def volume_changed(self):
+    def volume_changed(self, volume):
         """
         Called whenever the volume is changed.
 
         *MAY* be implemented by actor.
+
+        :param volume: the new volume in the range [0..100]
+        :type volume: int
         """
         pass
 

tests/core/listener_test.py

@@ -49,7 +49,7 @@ class CoreListenerTest(unittest.TestCase):
         self.listener.options_changed()
 
     def test_listener_has_default_impl_for_volume_changed(self):
-        self.listener.volume_changed()
+        self.listener.volume_changed(70)
 
     def test_listener_has_default_impl_for_seeked(self):
         self.listener.seeked(0)