[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/engine.py

@@ -56,6 +56,9 @@ class Module(object):
         """Return the widgets to draw for this module"""
         return self._widgets
 
+    def hidden(self):
+        return False
+
     def widget(self, name):
         for widget in self._widgets:
             if widget.name == name: