Move the list of contributors to a dedicated XObject

Manuel Leduc Thx for the contribution.

Some feedback. The documentation is a bit too light. It only describe the macro but:
1) it's missing explaining where the contributors are defined
2) it's missing an explanation about how to add contributors

And AFAICS we're also missing a UI to add contributors. That's a usage regression. It was easy to add them before but now the user of the app need to create a sub page with 2 xobjects filled properly. That's too technical. Maybe I've missed something though.

Thx