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