Works around a problem in Qt/Mac with mixed modal/non-modal dialogs.

Which leads to non-modal dialog getting incorrectly covered by the modal
one.

Task-number: QTCREATORBUG-5406
Change-Id: I1abd3051b9b849ecb5ee36316a0ab481ebcc754a
Reviewed-on: http://codereview.qt.nokia.com/1527


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>