table: Add render_header method to support custom table header. (#1746)

Closes #1733

## Example

```rs
impl TableDelegate for MyTable {
    fn render_header(&mut self, _: &mut Window, _: &mut Context<TableState<Self>>) -> Stateful<Div> {
        div().id("header").h_10()
    }
}
```

crates/ui/src/table/delegate.rs

@@ -35,6 +35,15 @@ pub trait TableDelegate: Sized + 'static {
     ) {
     }
 
+    /// Render the table head row.
+    fn render_header(
+        &mut self,
+        window: &mut Window,
+        cx: &mut Context<TableState<Self>>,
+    ) -> Stateful<Div> {
+        div().id("header")
+    }
+
     /// Render the header cell at the given column index, default to the column name.
     fn render_th(
         &mut self,
@@ -48,13 +57,15 @@ pub trait TableDelegate: Sized + 'static {
     }
 
     /// Render the row at the given row and column.
+    ///
+    /// Not include the table head row.
     fn render_tr(
         &mut self,
         row_ix: usize,
         window: &mut Window,
         cx: &mut Context<TableState<Self>>,
     ) -> Stateful<Div> {
-        h_flex().id(("row", row_ix))
+        div().id(("row", row_ix))
     }
 
     /// Render the context menu for the row at the given row index.

crates/ui/src/table/state.rs

@@ -865,7 +865,7 @@ where
             })
     }
 
-    fn render_table_head(
+    fn render_table_header(
         &mut self,
         left_columns_count: usize,
         window: &mut Window,
@@ -879,13 +879,18 @@ where
             self.fixed_head_cols_bounds = Bounds::default();
         }
 
-        h_flex()
+        let mut header = self.delegate_mut().render_header(window, cx);
+        let style = header.style().clone();
+
+        header
+            .h_flex()
             .w_full()
             .h(self.options.size.table_row_height())
             .flex_shrink_0()
             .border_b_1()
             .border_color(cx.theme().border)
             .text_color(cx.theme().table_head_foreground)
+            .refine_style(&style)
             .when(left_columns_count > 0, |this| {
                 let view = view.clone();
                 // Render left fixed columns
@@ -1298,7 +1303,7 @@ where
             .id("table-inner")
             .size_full()
             .overflow_hidden()
-            .child(self.render_table_head(left_columns_count, window, cx))
+            .child(self.render_table_header(left_columns_count, window, cx))
             .context_menu({
                 let view = cx.entity().clone();
                 move |this, window: &mut Window, cx: &mut Context<PopupMenu>| {