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/ui2/src/components/stories/list.rs

@@ -22,12 +22,12 @@ impl Render for ListStory {
             .child(Story::label("With sections"))
             .child(
                 List::new()
-                    .child(ListHeader::new("Fruits"))
+                    .header(ListHeader::new("Produce"))
+                    .child(ListSubHeader::new("Fruits"))
                     .child(ListItem::new("apple").child("Apple"))
                     .child(ListItem::new("banana").child("Banana"))
                     .child(ListItem::new("cherry").child("Cherry"))
                     .child(ListSeparator)
-                    .child(ListHeader::new("Vegetables"))
                     .child(ListSubHeader::new("Root Vegetables"))
                     .child(ListItem::new("carrot").child("Carrot"))
                     .child(ListItem::new("potato").child("Potato"))