Make SettingsDialog robust against multiple accept()/rejects()

That can happen if the user clicks very fast due to the multiple
levels of event loops.

Task-Nr: QTCREATORBUG-4818

Change-Id: I9c00dbf957a587c6e5839afebdd71e84b8c4a3c8
Reviewed-on: http://codereview.qt.nokia.com/16


Reviewed-by: Daniel Teske <daniel.teske@nokia.com>