Proper .oga file handling in FileRef

This fixes a problem introduced in r983337. OGA files are mostly likely going
to be Ogg::FLAC, if applications are following the Xiph recommendation. But
they can be using any Ogg codec, so we must check multiple formats (Sound Juicer
on Ubuntu used to produce .oga files for Ogg Vorbis, I believe it doesn't do that
anymore).


git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/kdesupport/taglib@1039708 283d02a7-25f6-0310-bc7c-ecb5cbfe19da