qml tool on OSX: wait for a timeout before exiting

Double-clicking to open a QML file was not working because it would
exit if no files are given on the command line.  It needs to wait
a while for the QFileOpenEvent.

Task-number: QTBUG-34926
Change-Id: Icb585a777b0438db85120c62e7717f0f6eafffb1
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>