Port the application menu to qqc2

Summary:
Move to an item-based qqc2 implementation.
The menu is the last blocker for a full qqc2 Elisa!

This must not land until the corresponding patch
for the qqc2 desktop style is landed released

BUG: 405144

Depends on D21944
Depends on D22011

Test Plan: menu works as before

Reviewers: mgallien, ngraham

Reviewed By: mgallien, ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D21943