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