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/storybook2/src/story_selector.rs

@@ -23,6 +23,7 @@ pub enum ComponentStory {
     Keybinding,
     Label,
     List,
+    ListHeader,
     ListItem,
     Scroll,
     Text,
@@ -44,6 +45,7 @@ impl ComponentStory {
             Self::Keybinding => cx.build_view(|_| ui::KeybindingStory).into(),
             Self::Label => cx.build_view(|_| ui::LabelStory).into(),
             Self::List => cx.build_view(|_| ui::ListStory).into(),
+            Self::ListHeader => cx.build_view(|_| ui::ListHeaderStory).into(),
             Self::ListItem => cx.build_view(|_| ui::ListItemStory).into(),
             Self::Scroll => ScrollStory::view(cx).into(),
             Self::Text => TextStory::view(cx).into(),