[core/themes] Add state themes

Each widget can now return a state using the method "state()". This
string is then used to look up a theme information which is used instead
of the default or module theme, if found.

see #23

bumblebee/output.py

@@ -18,6 +18,9 @@ class Widget(object):
         pass in the module name in every concrete module implementation"""
         self.module = module.name
 
+    def state(self):
+        return "state-default"
+
     def full_text(self):
         """Retrieve the full text to display in the widget"""
         if callable(self._full_text):