Draw a circle around the snapping point for visual aid.
Different snapping target method, switchable in the 3d view header menu in the Transform menu.
* Closest: Snaps the closest vertex to the point
* Median: Snaps the median of the selection to the point
* Center: Snaps the transform center to the point (this is different from median because you can use Cursor/Boundbox as center)
Fix a bug with constraints/snap handling.