Remove all windows before quitting

This gives all entities a chance of running `Drop::drop` which,
in turn, could cause them to spawn a critical task. For example,
we use this capability when a language server is dropped and we
need to asynchronously send a shutdown message.

crates/gpui/src/app.rs

@@ -1230,6 +1230,13 @@ impl MutableAppContext {
         self.remove_dropped_entities();
     }
 
+    pub fn remove_all_windows(&mut self) {
+        for (window_id, _) in self.cx.windows.drain() {
+            self.presenters_and_platform_windows.remove(&window_id);
+        }
+        self.remove_dropped_entities();
+    }
+
     fn open_platform_window(&mut self, window_id: usize, window_options: WindowOptions) {
         let mut window =
             self.cx