Only nodes in the same term

Here's a patch to be able to navigate between nodes attached to the same term. To work on this, I had to refactor the query building so that it's easily alterable. I also found a consistency bug that I fixed: if I'm right, "the next item of a given item should always have the given item as previous item".
I put it so that others can use it, but it might need to be cleaner to be committed here.