Update star's interpretation of left/top

To align with all other elements in Shoes 4, the star should use
left/top as the actual left/top, not the center of the element.

See #423 for discussion on it. This is a deviation from Shoes 3, but was
purposefully chosen as the simpler option for consistency going forward.

Fixes #423.

What do you think @PragTob @KCErb @wasnotrice? Can happily do
the opposite on arc if we want instead.