tree: remove hidden, unused workspaces

src/tree/workspace.rs

@@ -47,6 +47,10 @@ impl WorkspaceNode {
         self.container.set(Some(container.clone()));
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.stacked.is_empty() && self.fullscreen.get().is_none() && self.container.get().is_none()
+    }
+
     pub fn stacked_visible(&self) -> bool {
         self.visible.get() && self.fullscreen.get().is_none()
     }