Add a fallback for QLibraryInfo::DataPath as well.

This should remove the necessity to have a qt.conf file for deployed
Windows applications.

Also, be consistent about three dots in the messages that get printed
when trying fallback directories.

Task-number: QTBUG-45614
Change-Id: If46c93a0e8f150b91cff188517187a58edff9144
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>