Tab.menu should be Tab.contextMenu

Justification for change: Tab.menu is the odd one out when it comes to API for setting a context menu. This is because Tab is not a Control, and therefore doesn't inherit the contextMenu property from Control. Similarly, TableColumn is not a control but has the contextMenu property as expected. If Tab.menu is left in place, it will just look out of place, and at this stage it is a trivial fix - after shipping Presidio it is set in stone.