Revert "Make Widget::diff mutable"

This reverts commit 497ebcd0c3.

widget/src/scrollable.rs

@@ -399,8 +399,8 @@ where
         vec![Tree::new(&self.content)]
     }
 
-    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(std::slice::from_mut(&mut self.content));
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(std::slice::from_ref(&self.content));
     }
 
     fn size(&self) -> Size<Length> {