Add controls to Pick plugin to allow setting exact cutout

[ejeschke]

In testing #896, it occurred to me that it might be useful to provide controls for setting the exact location and radius of the cutout in the UI, instead of only setting this via a mouse drawing. This is then a proposal to add such controls. Because there are different cutout types, it might complicate the UI overmuch if we were to add parameters for all the aspects (e.g. all points for polygon, width and height for rectangle vs. radius for box, etc.) The UI is a bit crowded already. It might be sufficient for the majority of use cases to specify the center of the cutout, to allow an existing cutout to be re-positioned precisely.