dock: Add DockItem::panel (#464)

**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">

crates/ui/src/dock/dock.rs

@@ -188,6 +188,9 @@ impl Dock {
                     }
                 });
             }
+            DockItem::Panel { .. } => {
+                // Not supported
+            }
         }
     }
 
@@ -367,6 +370,7 @@ impl Render for Dock {
             .map(|this| match &self.panel {
                 DockItem::Split { view, .. } => this.child(view.clone()),
                 DockItem::Tabs { view, .. } => this.child(view.clone()),
+                DockItem::Panel { view, .. } => this.child(view.clone().view()),
             })
             .child(self.render_resize_handle(cx))
             .child(DockElement {

crates/ui/src/dock/mod.rs

@@ -68,17 +68,21 @@ pub struct DockArea {
 /// DockItem is a tree structure that represents the layout of the dock.
 #[derive(Clone)]
 pub enum DockItem {
+    /// Split layout
     Split {
         axis: gpui::Axis,
         items: Vec<DockItem>,
         sizes: Vec<Option<Pixels>>,
         view: View<StackPanel>,
     },
+    /// Tab layout
     Tabs {
         items: Vec<Arc<dyn PanelView>>,
         active_ix: usize,
         view: View<TabPanel>,
     },
+    /// Panel layout
+    Panel { view: Arc<dyn PanelView> },
 }
 
 impl DockItem {
@@ -140,6 +144,11 @@ impl DockItem {
         }
     }
 
+    /// Create DockItem with panel layout
+    pub fn panel(panel: Arc<dyn PanelView>) -> Self {
+        Self::Panel { view: panel }
+    }
+
     /// Create DockItem with tabs layout, items are displayed as tabs.
     ///
     /// The `active_ix` is the index of the active tab, if `None` the first tab is active.
@@ -192,6 +201,7 @@ impl DockItem {
         match self {
             Self::Split { view, .. } => Arc::new(view.clone()),
             Self::Tabs { view, .. } => Arc::new(view.clone()),
+            Self::Panel { view, .. } => view.clone(),
         }
     }
 
@@ -202,6 +212,7 @@ impl DockItem {
                 items.iter().find_map(|item| item.find_panel(panel.clone()))
             }
             Self::Tabs { items, .. } => items.iter().find(|item| *item == &panel).cloned(),
+            Self::Panel { view } => Some(view.clone()),
         }
     }
 
@@ -237,6 +248,7 @@ impl DockItem {
                     stack_panel.add_panel(new_item.view(), None, dock_area.clone(), cx);
                 });
             }
+            Self::Panel { .. } => {}
         }
     }
 
@@ -253,6 +265,7 @@ impl DockItem {
                     item.set_collapsed(collapsed, cx);
                 }
             }
+            DockItem::Panel { .. } => {}
         }
     }
 
@@ -261,6 +274,7 @@ impl DockItem {
         match self {
             DockItem::Tabs { view, .. } => Some(view.clone()),
             DockItem::Split { view, .. } => view.read(cx).left_top_tab_panel(true, cx),
+            DockItem::Panel { .. } => None,
         }
     }
 
@@ -269,6 +283,7 @@ impl DockItem {
         match self {
             DockItem::Tabs { view, .. } => Some(view.clone()),
             DockItem::Split { view, .. } => view.read(cx).right_top_tab_panel(true, cx),
+            DockItem::Panel { .. } => None,
         }
     }
 }
@@ -640,6 +655,9 @@ impl DockArea {
             DockItem::Tabs { .. } => {
                 // We subscribe to the tab panel event in StackPanel's insert_panel
             }
+            DockItem::Panel { .. } => {
+                // Not supported
+            }
         }
     }
 
@@ -707,6 +725,7 @@ impl DockArea {
         match &self.items {
             DockItem::Split { view, .. } => view.clone().into_any_element(),
             DockItem::Tabs { view, .. } => view.clone().into_any_element(),
+            DockItem::Panel { view, .. } => view.clone().view().into_any_element(),
         }
     }