QLoggingRegistry: Look up logging configuration in Qt data path

QLoggingRegistry: Look up logging configuration in Qt data path

Distributions like Fedora would like to disable logging globally, without
having to patch Qt. Fedora right now therefore adds a
/etc/xdg/qtlogging.ini file, which unfortunately though also messes with
Qt versions compiled by the user.

This patch lets QLoggingRegistry look up logging configurations also
in QLibraryInfo::DataPath, which would allow to tweak the values per
Qt installation.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1227295

Change-Id: I0fca304a47f45739d0c08a9e4e715673bf10aa80
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>