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/button/mod.rs

@@ -1,7 +0,0 @@
-mod button;
-mod button_like;
-mod icon_button;
-
-pub use button::*;
-pub use button_like::*;
-pub use icon_button::*;