[themes] Add "cycle" theme capability

It is now possible to add a list of theme configurations in the
"default" section called "cycle". These configuration items will be
cycled through module by module. to create "alternate style" effects.
This is *only* possible in the "default" configuration part, but any
module-specific configurations still take precedence.

Also, removed the capability of per-widget themes. That simply
complicates things and probably doesn't really bring any benefits.

bumblebee/output.py

@@ -1,5 +1,11 @@
 
 class Output(object):
+    def __init__(self, theme):
+        self._theme = theme
+
+    def theme(self):
+        return self._theme
+
     def start(self):
         pass