Add retained paints and shared text selection

examples/minimal.rs

@@ -15,7 +15,7 @@ impl DefaultAppState for State {
         rsc: &mut DefaultRsc<Self>,
         _: Proxy<Self::Event>,
     ) -> Self {
-        rect(Color::RED).set_root(rsc, &mut ui_state);
+        rect(PaintId::RED).set_root(rsc, &mut ui_state);
         Self { ui_state }
     }
 }