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/project_panel/src/project_panel.rs

@@ -4914,8 +4914,8 @@ impl Render for ProjectPanel {
                 )
                 .track_focus(&self.focus_handle(cx))
                 .child(
-                    uniform_list(cx.entity().clone(), "entries", item_count, {
-                        |this, range, window, cx| {
+                    uniform_list("entries", item_count, {
+                        cx.processor(|this, range: Range<usize>, window, cx| {
                             let mut items = Vec::with_capacity(range.end - range.start);
                             this.for_each_visible_entry(
                                 range,
@@ -4926,7 +4926,7 @@ impl Render for ProjectPanel {
                                 },
                             );
                             items
-                        }
+                        })
                     })
                     .when(show_indent_guides, |list| {
                         list.with_decoration(