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.rs

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