Read DSF header information

Split functions for reading integers from bytes out to a new private
file for the exclusive use of the tagging code.

https://bugzilla.gnome.org/show_bug.cgi?id=708368
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
Strip additional ID3 date fields while saving

Ensure that the year field in the UI is the only date field saved to an
ID3v2.3 tag.
WIP Add tests for DSF format support