Handle SelectionClientClose in QXcbClipboard

QXcbClipboard listens for subtype SelectionClientClose of Xfixes
SelectionNotify event, but doesn't handle it. When the client holding
the clipboard selection closes the Clipboard becomes empty and thus the
change should be emitted.

This fixes downstream KDE Bug #329174.

Change-Id: I19fb8cfd7bd3b249c0bc6ca2a724a9aeeb05ac7e
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>
Reviewed-by: Aleix Pol Gonzalez <aleixpol@kde.org>