[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/widget.py

@@ -5,6 +5,7 @@ class Widget(util.store.Store, core.input.Object):
     def __init__(self, full_text):
         super(Widget, self).__init__()
         self._full_text = full_text
+        self._module = None
 
     def full_text(self, value=None):
         if value:
@@ -14,4 +15,13 @@ class Widget(util.store.Store, core.input.Object):
                 return self._full_text(self)
         return self._full_text
 
+    def module(self, module):
+        self._module = module
+
+    def state(self):
+        rv = []
+        if self._module:
+            rv = self._module.state(self)
+        return rv if isinstance(rv, list) else [rv]
+
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