Allow sorting authors by first name

[Samuel Mehrbrodt]

From http://support.openlp.org/scp/tickets.php?id=2332

When editing authors in OpenLP all the authors are listed by their first name, in a single column. May another column might be added for last names.
Presently in our OpenLP we have 52 songs composed by men with the first name John, and this number will likely increase. When editing John Armstrong through to John Wimber those entries with second (third, or more) names determine the ordering of the authors, as per the attached image.
It would be great to search, sort, and edit by last names, as well as first.

[Samuel Mehrbrodt (sam92)]

Adding attachment from support issue