Menu, MenuItem: Make those guys QObjects, not QQuickItems

Added '__visualItem' private property to QtMenuBase, pointing to the
actual QtQuick item representing the menu item. We use this to get the
window an item belongs to, and its position inside the menu popup.

Also, moved some logic from ContextMenu into ComboBox, where it looks
more appropriate.

Change-Id: Idbed0497359457a696c7cac8703ae9d47f5e2003
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>