Add a parent_window event to Plasma Window interface
Summary:
This change adds a parent_window event to Plasma Window. From server
side it's possible to specify that a window is a transient for another
window - that is it has a parent window.
On client side this is exposed respectively with a new change signal.
Reviewers: Plasma on Wayland, hein
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D1779