macos: fix weird menubar rendering.

As app menu has been deprecated, the function of app menu should be merged into the menubar, which behaves like the original API of NSApp.

This also brings back the default app menu, which looks like native macOS apps.

For future refactoring, please note that the menubar must contain at least one menu before being set to NSApp to avoid weird menubar rendering.

Fixes #3967 (closed).