Add AsynchronousMessageBox

QMessageBox::warning is executing it's own event loop which leads to
crashes. Better is to use a message box which is using the main event
loop.

Change-Id: Ie05985dffcf505b25113755eece9b5e2256cd289
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: hjk <hjk121@nokiamail.com>