ext: Switch rest of bundled extensions to registry

mopidy/backends/stream/__init__.py

@@ -26,6 +26,6 @@ class Extension(ext.Extension):
     def validate_environment(self):
         pass
 
-    def get_backend_classes(self):
+    def setup(self, registry):
         from .actor import StreamBackend
-        return [StreamBackend]
+        registry.add('backend', StreamBackend)

mopidy/http/__init__.py

@@ -35,6 +35,6 @@ class Extension(ext.Extension):
         except ImportError as e:
             raise exceptions.ExtensionError('ws4py library not found', e)
 
-    def get_frontend_classes(self):
+    def setup(self, registry):
         from .actor import HttpFrontend
-        return [HttpFrontend]
+        registry.add('frontend', HttpFrontend)

mopidy/mpd/__init__.py

@@ -29,6 +29,6 @@ class Extension(ext.Extension):
     def validate_environment(self):
         pass
 
-    def get_frontend_classes(self):
+    def setup(self, registry):
         from .actor import MpdFrontend
-        return [MpdFrontend]
+        registry.add('frontend', MpdFrontend)