MenuStyle: Adding missing 'submenuPopupDelay' property

This amends e88bdffe644e53912dfbce95117555cb6a87bfd2
API-wise.

Change-Id: I9243aa05c3c4803e1c3ee08a761e782ee8e0841a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>