**Description**:
Improve the flexibility of `DockArea`, sometime user won't need a
`TabPanel`, just display some message or UI elements. Curren implement
only allow use Tab or Split. So this PR will add `Plain` to `DockItem`,
user can put anything which implement `PanelView` trait.
**Usage**:
```rust
let left_panels = DockItem::panel(Arc::new(<- PanelView ->));
_ = dock_area.update(cx, |view, cx| {
view.set_version(DOCK_AREA.version, cx);
view.set_left_dock(left_panels, Some(px(260.)), true, cx);
view.set_center(dock_item, cx);
view.set_dock_collapsible(
Edges {
left: false,
..Default::default()
},
cx,
);
});
```
**Screenshot**
<img width="1300" alt="image"
src="https://github.com/user-attachments/assets/a01e427c-50f1-4509-82d1-76cc016aa7b3">
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