Merge pull request #12769 from Profpatsch/beets

Updates beets to version 1.3.16, which comes with new plugins
"embyupdate", "edit" and "mbsubmit". See the following URL for a
detailed upstream changelog:

http://beets.readthedocs.org/en/v1.3.16/changelog.html

The "mbsubmit" plugin isn't listed there and made it more or less
silently into the release, see beetbox/beets#1779 for the final work on
the plugin.

Tested this locally with a few queries and using the new "edit" plugin.