Ensure focus_in and focus_out fire on window activation (#3993)

Also:
- Rename cx.on_blur to cx.on_focus_lost
- Fix a bug where notify calls in focus handlers were ignored
- Fix a bug where vim would get stuck in the wrong mode when switching
  windows

Release Notes:

- (preview only) vim: fix switching between multiple windows

crates/gpui/src/app/test_context.rs

@@ -186,6 +186,7 @@ impl TestAppContext {
         drop(cx);
         let view = window.root_view(self).unwrap();
         let cx = Box::new(VisualTestContext::from_window(*window.deref(), self));
+        cx.run_until_parked();
         // it might be nice to try and cleanup these at the end of each test.
         (view, Box::leak(cx))
     }