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/picker/src/picker.rs

@@ -17,7 +17,7 @@ use gpui::{
 use head::Head;
 use schemars::JsonSchema;
 use serde::Deserialize;
-use std::{sync::Arc, time::Duration};
+use std::{ops::Range, sync::Arc, time::Duration};
 use ui::{
     Color, Divider, Label, ListItem, ListItemSpacing, Scrollbar, ScrollbarState, prelude::*, v_flex,
 };
@@ -760,14 +760,13 @@ impl<D: PickerDelegate> Picker<D> {
 
         match &self.element_container {
             ElementContainer::UniformList(scroll_handle) => uniform_list(
-                cx.entity().clone(),
                 "candidates",
                 self.delegate.match_count(),
-                move |picker, visible_range, window, cx| {
+                cx.processor(move |picker, visible_range: Range<usize>, window, cx| {
                     visible_range
                         .map(|ix| picker.render_element(window, cx, ix))
                         .collect()
-                },
+                }),
             )
             .with_sizing_behavior(sizing_behavior)
             .when_some(self.widest_item, |el, widest_item| {