Menu: Enable mnemonic menu navigation

We also added a new mnemonic specific shortcut context matcher. This
prevents two menu items with the same mnemonic but within different
menus to be reported as ambiguous.

Task-number: QTBUG-33030
ChangeLog: Added mnemonic navigation for menus
Change-Id: I192c9aacba4d15851fe65bf9201251962fe976d5
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>