Rework ListHeader to be more open (#3467)

This PR reworks the `ListHeader` component to be more open.

The `meta` method can now be used to append meta items of any element to
the `ListHeader`, and they will be rendered with the appropriate spacing
between them.

Release Notes:

- N/A

crates/collab_ui2/src/collab_panel.rs

@@ -2511,7 +2511,7 @@ impl CollabPanel {
                 } else {
                     el.child(
                         ListHeader::new(text)
-                            .when_some(button, |el, button| el.right_button(button))
+                            .when_some(button, |el, button| el.meta(button))
                             .selected(is_selected),
                     )
                 }