Add window shade support to protocol, interfaces and test.
Summary:
This adds support for 'shadable' and 'shaded' states to the protocol and
to the client and server classes, as well as the window model.
Test Plan: The PlasmaWindowModel test has been extended to test the new states.
Reviewers: graesslin
Subscribers: plasma-devel
Projects: Plasma
Differential Revision: https://phabricator.kde.org/D1417