Add new author type "Altered by"

[smehrbrodt]

Requested here: https://bugs.launchpad.net/openlp/+bug/1546530

[gyuris]

OK, lets standardise this author type. "Altered by" is a good label for OpenLP, but it doesn't fit other types. Other author types are currently:

• words
• music
• translation (+language)

They answers the question: What this author produced? So I suggest: alteration, variant, revision...

We have currently a <variant> tag in metadata, see in documentation. It suggest variant. It would be clear that if a song has a variant that we can assign an author with variant type:

<variant>Newsboys</variant>
<authors>
  <author type="words">XY</author>
  <author type="music">XY</author>
  <author type="variant">Michael Tait</author>
</authors>