Update libwnck "active-window-changed" callback due to API changes

shell/model/homemodel.py

@@ -182,7 +182,7 @@ class HomeModel(gobject.GObject):
     def _set_active_error(self, err):
         logging.error("set_active() failed: %s" % err)
 
-    def _active_window_changed_cb(self, screen):
+    def _active_window_changed_cb(self, screen, previous_window=None):
         window = screen.get_active_window()
         if window is None:
             return

shell/view/frame/eventarea.py

@@ -101,6 +101,6 @@ class EventArea(gobject.GObject):
         for window in self._windows:
             window.hide()
 
-    def _active_window_changed_cb(self, screen):
+    def _active_window_changed_cb(self, screen, previous_window=None):
         for window in self._windows:
             window.window.raise_()