Add destroy() to playback provider API

mopidy/backends/base/playback.py

@@ -68,10 +68,8 @@ class BasePlaybackController(object):
     def destroy(self):
         """
         Cleanup after component.
-
-        May be overridden by subclasses.
         """
-        pass
+        self.provider.destroy()
 
     def _get_cpid(self, cp_track):
         if cp_track is None:
@@ -496,6 +494,14 @@ class BasePlaybackProvider(object):
     def __init__(self, backend):
         self.backend = backend
 
+    def destroy(self):
+        """
+        Cleanup after component.
+
+        May be overridden by subclasses.
+        """
+        pass
+
     def pause(self):
         """
         To be overridden by subclass. Implement your backend's pause