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.
Sample fixes for change in left/top for star
Method extractions and clarifying comments
Respect center option on star
Merge pull request #993 from shoes/star-left-top

Start now interprets left/top as upper left corner, like all elements

* This derviates from shoes3 behavior but the decision was taken for consistency reasons, you can use the center: true options to get vintage behavior
* Changelog: breaking