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