ThumbnailPicker Should Have Public Interface To Show Desired Thumb

[guest]

There currently seems to be no way to tell the ThumbnailPicker? to display a particular thumbnail via a public interface. Internally it can (obviously) do it already as it uses such functionality to page between thumbs, so hopefully it should be pretty straightforward to expose some of this functionality to the public, too. It'd enable augmented interfaces to the picker (things like page forward, page back, jumps to particular images, and so on) plus smarter interfaces (like a picker that had a memory of the prior pick and started at that location).