Use AtomLayoutResponse in TextEditOutput (#8003)

This is necessary to add e.g. buttons as prefix/suffix within the
textedit (so you can read the rect and place the button).

I'm not sure if deref on the AtomLayoutResponse is the right call but it
should make the migration easier and is generally convenient (but might
lead to some confusion).

crates/egui/src/atomics/atom_layout.rs

@@ -520,6 +520,20 @@ impl AtomLayoutResponse {
     }
 }
 
+impl Deref for AtomLayoutResponse {
+    type Target = Response;
+
+    fn deref(&self) -> &Self::Target {
+        &self.response
+    }
+}
+
+impl DerefMut for AtomLayoutResponse {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.response
+    }
+}
+
 impl Widget for AtomLayout<'_> {
     fn ui(self, ui: &mut Ui) -> Response {
         self.show(ui).response