Revert "Make Widget::diff mutable"

This reverts commit 497ebcd0c3.

widget/src/combo_box.rs

@@ -509,7 +509,7 @@ where
         vec![widget::Tree::new(&self.text_input as &dyn Widget<_, _, _>)]
     }
 
-    fn diff(&mut self, _tree: &mut widget::Tree) {
+    fn diff(&self, _tree: &mut widget::Tree) {
         // do nothing so the children don't get cleared
     }