Add Close button to window asking what to do with changed documents

Add a close button to window asking what to do with documents that
were changed outside Creator. Finally a simple way to get rid of
open moc-files that trigger this window over and over again:-)

Task-number: QTCREATORBUG-7360
Change-Id: I7a373ed60d85d810caf6604bc7dae7bb7fa186a9
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>