Make Widget::layout and operate mutable

... and bless `responsive`!

widget/src/container.rs

@@ -259,7 +259,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -273,12 +273,14 @@ where
             self.padding,
             self.horizontal_alignment,
             self.vertical_alignment,
-            |limits| self.content.as_widget().layout(tree, renderer, limits),
+            |limits| {
+                self.content.as_widget_mut().layout(tree, renderer, limits)
+            },
         )
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -288,7 +290,7 @@ where
             self.id.as_ref().map(|id| &id.0),
             layout.bounds(),
             &mut |operation| {
-                self.content.as_widget().operate(
+                self.content.as_widget_mut().operate(
                     tree,
                     layout.children().next().unwrap(),
                     renderer,