Revert "Make Widget::diff mutable"

This reverts commit 497ebcd0c3.

widget/src/text_input.rs

@@ -654,7 +654,7 @@ where
         tree::State::new(State::<Renderer::Paragraph>::new())
     }
 
-    fn diff(&mut self, tree: &mut Tree) {
+    fn diff(&self, tree: &mut Tree) {
         let state = tree.state.downcast_mut::<State<Renderer::Paragraph>>();
 
         // Stop pasting if input becomes disabled