[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

tests/util.py

@@ -28,10 +28,14 @@ class MockOutput(object):
     def end(self):
         pass
 
-class MockWidget(object):
+class MockWidget(Widget):
     def __init__(self, text):
         self._text = text
         self.module = None
+        self.attr_state = "state-default"
+
+    def state(self):
+        return self.attr_state
 
     def update(self, widgets):
         pass