add pid to plasma window management protocol
Summary:
This patch adds a pid event to the plasma window management protocol. It
allows the compositor to tell allow a mapping between windows and processes.
Bumps the version number of the interface to 8 to indicate this.
Test Plan: autotest added, passed
Reviewers: Plasma, hein, graesslin
Reviewed By: Plasma, hein, graesslin
Subscribers: apol, davidedmundson, plasma-devel, Frameworks
Tags: Frameworks, Plasma on Wayland
Differential Revision: https://phabricator.kde.org/D5747