Use MenuItem's action property directly in ActionMenuItem

Summary:
Now that Kirigami's Action is a QQC2 Action subclass we can use
MenuItem's action property directly, removing the need for custom
bindings.

This fixes Kirigami ActionMenus not showing keyboard shortcuts for
actions, as that binding was missing.

Test Plan:
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

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D24501