tree: remove hidden, unused workspaces

src/tree/output.rs

@@ -180,6 +180,10 @@ impl OutputNode {
             }
             collect_kb_foci2(old.clone(), &mut seats);
             old.set_visible(false);
+            if old.is_empty() {
+                old.clear();
+                self.state.workspaces.remove(&old.name);
+            }
         }
         ws.set_visible(true);
         if let Some(fs) = ws.fullscreen.get() {

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()
     }