[core] Add (partial) support for states

Add states to the modules and widgets. Widgets are mostly just a
pass-through (backwards compatibility, and ease of use - making states
directly inside the widgets would require more code inside the modules
to ensure that each widget is correctly updated).

Still missing:
- Separators during partial update (right now, it takes one interval
until separators are drawn correctly)

core/output.py

@@ -78,6 +78,8 @@ class i3(object):
 
     def update(self, affected_modules=None):
         for module in self._modules:
+            if affected_modules and not module.id() in affected_modules:
+                continue
             module.update()
             self._status[module] = self.widgets(module)