Hide the toolbar if it has no visible items (#3654)

This PR makes the toolbar hide itself if it has no visible items.

This removes the double border beneath the tab bar when there are no
visible tools in the toolbar.

Release Notes:

- N/A

crates/workspace2/src/toolbar.rs

@@ -55,6 +55,12 @@ pub struct Toolbar {
 }
 
 impl Toolbar {
+    fn has_any_visible_items(&self) -> bool {
+        self.items
+            .iter()
+            .any(|(_item, location)| *location != ToolbarItemLocation::Hidden)
+    }
+
     fn left_items(&self) -> impl Iterator<Item = &dyn ToolbarItemViewHandle> {
         self.items.iter().filter_map(|(item, location)| {
             if *location == ToolbarItemLocation::PrimaryLeft {
@@ -90,6 +96,10 @@ impl Render for Toolbar {
     type Element = Div;
 
     fn render(&mut self, cx: &mut ViewContext<Self>) -> Self::Element {
+        if !self.has_any_visible_items() {
+            return div();
+        }
+
         let secondary_item = self.secondary_items().next().map(|item| item.to_any());
 
         v_stack()