Expose application menu via KWindowInfo
Summary: Application menu information is now available via KWindowInfo's APIs on X11.
Reviewers: Plasma, broulik, zzag, KWin
Reviewed By: Plasma, zzag, KWin
Subscribers: zzag, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D28112