When building menu hyerarchies, parent menus to their containers

Summary:
Wayland requires menu windows to have a "transient parent" as wl_shell
protocol refuses to either position correcty or assign the corrent
flags to popup windows that don't have a parent.
a lot of popup menus in KDE applications don't have any parent
and will look wrong in wayland. this fixes at least some of the occurrences
of popup menus build by kxmlgui

Test Plan:
"Bookmarks" menu in konsole menubar is now correct in
wayland

Reviewers: Plasma, Plasma on Wayland, Frameworks, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5806