Add priority seating

[Robbendebiene]

Resolves #898

[skinkie]

I like your contributions... but could you have started with this like two months ago?

[Robbendebiene]

At the time we didn't really know what the process for contributing to NeTEx was. So we first created the proposals as issues and waited for feedback. Now knowing more about the process and especially the 2.0 deadline we would have pushed harder.

Edit: Just to be clear, we do not expect these changes to land in 2.0. In the last meeting we more or less agreed on 2.1

[Aurige]

That's a nice addition but we can't add it in the current next since we cannot update the 2.0 documentation accordingly anymore