Moves this to the start of the previous visible line.
Returns true if this could be moved; i.e. if this was at character offset 0, this function returns false. Therefore if this was already on line 0, but not at the start of the line, this is snapped to the start of the line and the function returns true. (Note that this implies that in a loop calling this function, the line number may not change on every iteration, if your first iteration is on line 0.)
whether this moved