contrib: pick: make the tree graphics a proper part of the format

contrib: pick: make the tree graphics a proper part of the format

Previously the box graphics in the pick view were always attached to
the subject. Make them a field in their own right. We use the
recursive insert to change the default notmuch-pick-result-format so
that the user view does not change. (The subject touches the tree box
graphics but the next column (tags) is still vertically aligned.)