[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/module.py

@@ -20,6 +20,8 @@ class Module(core.input.Object):
         super().__init__()
         self._config = config
         self._widgets = widgets if isinstance(widgets, list) else [ widgets ]
+        for widget in self._widgets:
+            widget.module(self)
         self._name = None
 
     def parameter(self, key, default=None):
@@ -45,6 +47,9 @@ class Module(core.input.Object):
     def widgets(self):
         return self._widgets
 
+    def state(self, widget):
+        return []
+
 class Error(Module):
     def __init__(self, config, module, error):
         super().__init__(config, core.widget.Widget(self.full_text))