gpui: Simplify uniform list API by removing entity param (#32480)

This PR also introduces `Context::processor`, a sibling of
`Context::listener` that takes a strong pointer to entity and allows for
a return result.

Release Notes:

- N/A

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/gpui/src/app/context.rs

@@ -225,6 +225,18 @@ impl<'a, T: 'static> Context<'a, T> {
         }
     }
 
+    /// Convenience method for producing view state in a closure.
+    /// See `listener` for more details.
+    pub fn processor<E, R>(
+        &self,
+        f: impl Fn(&mut T, E, &mut Window, &mut Context<T>) -> R + 'static,
+    ) -> impl Fn(E, &mut Window, &mut App) -> R + 'static {
+        let view = self.entity();
+        move |e: E, window: &mut Window, cx: &mut App| {
+            view.update(cx, |view, cx| f(view, e, window, cx))
+        }
+    }
+
     /// Run something using this entity and cx, when the returned struct is dropped
     pub fn on_drop(
         &self,