[theme] Fix cycling through widget styles

Cycled widget styles (such as the battery charging style) were broken
until now. The reason for this: They maintain state that represents the
current cycle position (i.e. what is the current icon that is being
displayed), but that is done in a way that uses repr() on the widget
object.

Since the widget objects are re-created each time the bar is drawn, this
is a deeply flawed design.

Instead, use the instance() of the widget for now.

bumblebee/config.py

@@ -78,6 +78,10 @@ class Config(object):
         return self._store.get(name, default)
 
     def increase(self, name, limit, default):
+        if not name in self._store:
+            self._store[name] = default
+            return default
+
         self._store[name] += 1
         if self._store[name] >= limit:
             self._store[name] = default