Make use of foreign protocol

Summary:
not sure if that is the right "parent" that has
to be set, set the transient for surfaces that
have setParentOf of an imported set

Test Plan:
m_XdgForeign->transientFor(surface()) finds the surface is expected,
not sure how to test it further

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: luebking, graesslin, davidedmundson, plasma-devel, kwin, KWin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7521