Fix opening of folds containing other folds.

Before we were removing the folded attribute from all
characters in the parent’s range, opening nested folds as
well.

Opening multiple fold levels at once isn’t quite working yet.
Only exclude closed child folds from being unmarked as folded.

This when closing a parent fold. Before, we would
indiscrimately skip child folds when removing the folded
attribute, even if the child fold had never been closed. This
led to some interesting-looking results, since the first
character of the child fold had never been marked with the
folded attachment, so you just ended up with the first
character of the child fold and none of the other characters
therein.
Recursively *open* folds when doing openFold: with >1 levels.

We had accidentally been invoking closeFold:levels: when
recursing into children. Whoops.