MessageBox exposes About Avalonia in the macOS application menu

On macOS, showing a MessageBox replaces the application's own menu bar with Avalonia's default application menu. The application's File, Edit and Help menus disappear while the message box is open, and an About Avalonia command becomes available. Choosing it opens Avalonia's framework information window instead of the application's own About behaviour.

Keep the application's appropriate menu and modal command behaviour while the dialog is displayed, without exposing a framework default menu. The normal application menu returns after the message box is dismissed.