Improve the deconstruction of PlasmaWindows

Summary:
The protocol is extended by a dedicated destructor request. When a
PlasmaWindow is umapped we no longer destroy the resource directly,
but only send the unmap. The client is then supposed to clean up
(which it already did in that case) and will invoke the destructor.

The PlasmaWindowInterface object will be automatically deleted after
the unmap once all resources bound for it are destroyed.

The tests are extended by two new test cases which triggered protocol
errors on the client side prior to this change.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1594