Open popup window when receiving a call

We still need to style and allow people to accept the call but this
is a good starting point.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/app.rs

@@ -786,6 +786,10 @@ impl AsyncAppContext {
         self.update(|cx| cx.add_window(window_options, build_root_view))
     }
 
+    pub fn remove_window(&mut self, window_id: usize) {
+        self.update(|cx| cx.remove_window(window_id))
+    }
+
     pub fn platform(&self) -> Arc<dyn Platform> {
         self.0.borrow().platform()
     }