[server] Properly handle the case that a PlasmaWindow is unmapped before client bound it

Summary:
So far we directly destroyed the resource on the server side. But this
causes a wayland error when the client tries to cleanup. This results in
the client being destroyed. A problem which brings down plasmashell
regularly when short living windows are shown. This happens e.g. in
Dolphin with the adress auto complete.

This change addresses the problem by creating a temporary
PlasmaWindowInterface for the already unmapped window. It doesn't get
added to the list of known windows and it's only purpose is to properly
handle the unmap and the destroy of the just created resource.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1734