New global USE flag "id3tag"

id3tag: Enable support for ID3 metadata

Current consumers:

USE="id3tag":

media-sound/alsaplayer: Enables ID3 tagging with id3tag library
media-sound/audacity: Enables ID3 tagging with id3tag library
media-sound/mpd: Support for ID3 tags
media-sound/sonic-visualiser: Enables ID3 tagging with id3tag library
media-sound/sox: Enables ID3 tagging with id3tag library
media-video/vlc: Enables id3tag metadata reader plugin.

USE="id3" which will be renamed to USE="id3tag" wrt #421873:

media-sound/abcde: Support ID3, ID3v2 tagging of audio files