[NTG-context] collecting user wishes for streams related functionalities

[Taco Hoekwater]

> 
> To extrapolate this facility, if columns on one page can run such
> parallel texts then we can have multiple translations running together.

I would like to do something slightly more general: it would be nice if
columnset blocks could take their input from streams, even if the block
is not a full column (or multiple columns).

Best wishes,
Taco