[O] Feature suggestion: VISIBILITY: n children

[Muchenxuan Tong]

Right now for 'VISIBILITY' property Org has the following options: 
folded, children, content, all

I think one more flexible options--n children--would be useful (n is a 
integer)

It restricts how many top children you can see under the current headline.

This is inspired by Things for Mac app, where in the "Next" view, for 
each project, you only see the 3 immediate todos. I want the same thing 
in my Org work flow but found no straightforward solution.