Make Widget::layout and operate mutable

... and bless `responsive`!

widget/src/pin.rs

@@ -139,7 +139,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -151,7 +151,7 @@ where
 
         let node = self
             .content
-            .as_widget()
+            .as_widget_mut()
             .layout(tree, renderer, &layout::Limits::new(Size::ZERO, available))
             .move_to(self.position);
 
@@ -160,13 +160,13 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn widget::Operation,
     ) {
-        self.content.as_widget().operate(
+        self.content.as_widget_mut().operate(
             tree,
             layout.children().next().unwrap(),
             renderer,