Stop using stale timestamps (x11)
QX11Info::getTimestamp() is not a reliable timestamp source for
our precious needs as it has an undocumented (but known - see
QTBUG-56595) behavior that it reports a timestamp from an
earlier call if previous reply of the server is still in the event
queue.
This new one is based on Qt's QXcbConnection::getTimestamp() but
always returns the latest timestamp.
Edited by Roland Pallai