2 bugs in SelectionSpecify

Hi all, I'd like to submit a fix to SelectionSpecify that corrects two problems:
* The ImageDisplay is not marked with ItemIO.Both, so CellProfiler neglects to harvest the display after the overlay has been added.
* The origin of the circle overlay is its center but the origin of the rectangle overlay is the left-top corner - this caused the circle to be centered when "center" was checked. I've made it work - a bit clumsily due to the differing interpretations of origin.

Hope I got it right, please check over and accept if I have.