Designer: Allow stroke offset on non-closed lines

[dock]

I really like the ability to offset the stroke to the inside, outside or centre of the stroke.

 

Unfortunately, I noticed that it only works on closed strokes!  I understand that a single stroke doesn't have a concept of inside and outside, but the functionality is still very useful.

 

Even if it misunderstood which was the inside and outside, and even if that got flipped when I closed the loop, it would still be very useful.