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