Switch ThumbnailItem to internal uuid instead of WId

Summary:
So far the ThumbnailItem in TabBox mode used the window id for finding
the window it should render a thumbnail on. In the Wayland world this is
not unique. The window id could be either an X11 window or a wayland
window. We don't guarantee that there are no conflicting ids.

With the internal id we have a way to properly identify the windows, so
this element should use them.

To support this the property changed the type to QUuid and the
clientmodel also provides the QUuid. As in TabBox the way to get the
window is through the model this should be compatible for all themes.
It's tested and verified with the Breeze switcher.

For declarative KWin scripts the ThumbnailItem also provides the
AbstractClient as a property, so there should not be any script which
uses wid. If it does, this could break, but well the script should use
the intended API.

Test Plan: ctest passes, manual testing of Breeze alt-tab switcher

Reviewers: #kwin

Differential Revision: https://phabricator.kde.org/D18405