ui/cocoa: Add Services menu

Services menu functionality of Cocoa is described at:
https://developer.apple.com/design/human-interface-guidelines/macos/extensions/services/



Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220214091320.51750-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>