[Kst] Advance one screen

[Brisset, Nicolas]

> I agree with Barth.
Yes, command line usage and widget selection should be differentiated. 

> I don't why it whould go automagically into 'read to end' 
> mode when you reach the end of a vector.
Neither do I. Moving page-wise through the data should be consistent
between both directions (forward and backward). If we change to 'read to
end' when reaching the end of data, then moving back one screen would
not have the expected behavior.

> There is a button in the bar for that.
So, really no reason to do it automatically. If that's what the user
wants at some point, it's just one click away...