QX11Info: add nextStartupId/setNextStartupId - for startup-notification

This controls what the next window to be shown will send as startup-notification
message.
http://standards.freedesktop.org/startup-notification-spec/startup-notification-latest.txt

Change-Id: Ia1b5e6ba004d61ca6fdfd6683d2e22dfcd0c42fb
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Richard J. Moore <rich@kde.org>