Make widget::Tree mutable in Widget::layout

widget/src/overlay/menu.rs

@@ -232,7 +232,7 @@ where
     Renderer::Theme: StyleSheet + container::StyleSheet,
 {
     fn layout(
-        &self,
+        &mut self,
         renderer: &Renderer,
         bounds: Size,
         position: Point,
@@ -352,7 +352,7 @@ where
 
     fn layout(
         &self,
-        _tree: &Tree,
+        _tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {