Read ID3v2 tags from DSF files

Split off a helper et_id3tag_fill_file_tag_from_id3tag() function from
id3tag_read_file_tag() and use it to read the parsed id3_tag struct. Add
a private header to share some defines between the header and tag
reading code.

https://bugzilla.gnome.org/show_bug.cgi?id=708368
Write ID3v2.4 tags to DSF files

https://bugzilla.gnome.org/show_bug.cgi?id=708368
Write ID3v2.3 tags to DSF files

https://bugzilla.gnome.org/show_bug.cgi?id=708368
Move private ID3 functions into a separate header