And restore proper scrolling in all the other lower notebook items under gtk3

it seems like scrolling in a sourceview whose buffer has just been modified is
not the easiest thing to do in gtk 

replaces GToolbox.select_file that no longer exists in lablgtk3

(See [this discussion](https://github.com/garrigue/lablgtk/issues/2#issuecomment-451203335) upstream for more information)

Also adds some comments.

On nested loops, for each iteration of the outer loop, postpone the widening of
the inner loop of [delay] iterations, [delay] being set by -eva-widening-delay.

Example:

while (i < n) {
  j = 0;
  while (j < 100) { j++; }
  i++;
}

On such nested loops, the fixpoint of the inner loop is independent from the
outer loop.

At the first iteration of the outer loop, a fixpoint is reached for the inner
loop. For other iterations of the outer loop, only one iteration of the inner
loop is needed to reach a new fixpoint, by only updating the variables modified
by the outer loop (i here). Avoiding widening during this one iteration is
crucial for precision, as the widening of the inner loop would widen the
variable i, leading to an overflow alarm at i++.