add module system and move sensor into core with it

src/layout/id.rs

@@ -87,6 +87,10 @@ impl<W> WidgetId<W> {
         unsafe { std::mem::transmute(self) }
     }
 
+    pub fn key(&self) -> Id {
+        self.id.duplicate()
+    }
+
     pub(super) fn cast_type<W2>(self) -> WidgetId<W2> {
         // safety: self is repr(C) and generic only used for phantom data
         unsafe { std::mem::transmute(self) }