Ensure that the user codecs are listed in QTextCodec::availableCodecs

Codecs registered by creating new QTextCodec instances should be listed
there.

Task-number: QTBUG-32500
Change-Id: I56c00e0d6bbfef55a6cbd571bcf9aa2cf333ef3a
Reviewed-by: David Faure <david.faure@kdab.com>