[core] Add ability to "hide" a module with all its widgets

* A module can override the method "hidden" to specify when all it's
widgets should be hidden.
* Implement hidden for cmus and spotify
* Fix problem that cmus widgets were not displayed correctly

fixes #157

bumblebee/modules/spotify.py

@@ -40,6 +40,9 @@ class Module(bumblebee.engine.Module):
     def spotify(self, widget):
         return str(self._song)
 
+    def hidden(self):
+        return str(self._song) == ""
+
     def update(self, widgets):
         try:
             bus = dbus.SessionBus()