Set a transient parent for SNI context menus
Summary:
Those had no transient parent set, so they got displayed somewhere, most of the
time on the wrong screen.
Test Plan:
Works fine for spotify and kteatime now, but certain applications
which trigger a LayoutChanged signal on the opened event have a Y offset.
Reviewers: Plasma
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D11584