use transientparent directly

Summary:
don't parent directly as we don't want unwanted deletions.
ensure we have a menu, not parented yet, the container exists,
both the menu and the container's toplevel widget QWindows exist,
then set the transient parent of the qmenu's qwindow to the
container's toplevel widget's qwindow. this fixes positioning in
wayland as the protocol requires every popup menu having a transient parent

Test Plan:
okular doesn't crash anymore, konsole's bookmarks menu still
correctly positioned in wayland

Reviewers: lbeltrame

Reviewed By: lbeltrame

Subscribers: lbeltrame, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5900