easytag: Update to version 2.1.8.

Easytag has moved to gnome.org and thus this commit also updates and cleans up a
few meta attributes. More information about the move can be found in the
announcement:

https://mail.gnome.org/archives/easytag-list/2012-November/msg00006.html

In order to get it to compile, we need to do a bit of patching, for example the
configure script tries to find libid3tag through pkg-config, but unfortunately
libid3tag doesn't have a *.pc script, so we're patching it out of the configure
script and use NIX_LDFLAGS to inject the library during linking (note the "-lz"
- it's a propagated dependency of libid3tag).

Also added for MP4 support: taglib.

Thanks to @devhell for the notification of the new upstream release.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>