Keep existing Overlays

Since I want to keep the results (overlays) from a previous computation step I have removed the explicit clearing of an existing ```Overlay``` in the active ```ImagePlus```. This does not seem to raise any obvious issues; unit tests are passed.

Furthermore, I have shifted the origin of ```SpotOverlay``` and ```TrackOverlay``` out of the visible region of the image. Although it's not a clean solution, it does hide the labels of the aforementioned overlays which are otherwise located in the upper left corner at (0, 0).