[core] Add preliminary block abstraction to output

The idea is to simplify the way the output module currently works by:
- introducing an abstraction that represents blocks; these abstractions
  contain all data - uninterpreted - required to draw a block
- separately from that, whenever the block is serialized into JSON,
  do the interpretation (pango vs. non-pango, etc.)

This - theoretically - should simplify code by creating two separate
concerns: collecting the data and actually interpreting it.

core/theme.py

@@ -88,6 +88,9 @@ class Theme(object):
         self.__previous = dict(self.__current)
         self.__current.clear()
 
+    def get(self, key, widget=None, default=None):
+        return self.__get(widget, key, default)
+
     def __get(self, widget, key, default=None):
         if not widget:
             widget = core.widget.Widget('')