count visible lines forward, if possible (if count would move
past the start or end of the buffer, moves to the start or end of
the buffer). The return value indicates whether the iterator moved
onto a dereferenceable position; if the iterator didn't move, or
moved onto the end iterator, then FALSE is returned. If count is 0,
the function does nothing and returns FALSE. If count is negative,
moves backward by 0 - count lines.gtkTextIterForwardVisibleLines(object, count)objectcountiter moved and is dereferenceable