Custom Center for the Vignetting Tool

Add an option to select the center for the vignetting tool.

If you take a half body shot, but you might want the face of the subject to be the center of the vignette so that the eye of the viewer is subtly directed toward the face. At the moment you need rely on the radial gradient tool, it would be great to have the possibility to add a custom center but just clicking where you want the center to be, and use the existing slider to adjust the vignette intensity