the title tracker list, and it wastes more memory.
But it makes implementing list command elegant, since we've just visit tags,
then print out the visited tags in tag tracker (which has the benefit of
making sure everything is in sorted order)

git-svn-id: https://svn.musicpd.org/mpd/trunk@2608 09075e82-0dd4-0310-85a5-a0d7c8717e4f