Use MenuItem's action property directly in ActionMenuItem
Now that Kirigami's Action is a QQC2 Action subclass we can use
MenuItem's action property directly, removing the need for custom
This fixes Kirigami ActionMenus not showing keyboard shortcuts for
actions, as that binding was missing.
Applications' overflow menus and action submenus still work correctly.
Menu items for actions that have keyboard shortcuts now display them.
Reviewers: Kirigami, mart
Reviewed By: Kirigami, mart
Subscribers: apol, plasma-devel
Differential Revision: https://phabricator.kde.org/D24501