Added some test cases for FileRef about unsupported files
Added a missing check if an Opus file is open
Changed the style to follow TagLib's one
Comment update: Added some supplementary explanations and fixed some spellings.
Added conversion from String to const wchar_t*
Updated the related comments
Added the methods to check which kind of tags the file actually has.
Updated the relevant comments
Merge remote-tracking branch 'TsudaKageyu/has-tags'
Merge remote-tracking branch 'TsudaKageyu/comment-update'
Merge remote-tracking branch 'TsudaKageyu/fix-opus'
Merge remote-tracking branch 'TsudaKageyu/string-api-master'
Merge branch 'master' into taglib2

Conflicts:
	taglib/ogg/opus/opusfile.cpp
	taglib/riff/wav/wavfile.cpp
	taglib/toolkit/tstring.h