Enable refreshing the image without deleting annotations

[hajicj]

We may want to edit the underlying image (e.g., local binarization) as annotation progresses, without reloading it and losing the annotations all the time.

There should be some "destructivity flag" or auto-check in App.import_image() - perhaps check if the new image dimensions are different? This will have very few false positives, I'd say, and if necessary, the user can just use the Clear Cropobjects button.

[hajicj]

Note that the image dimension heuristic does not work well when there is a series of images from the same camera.