U4-4989: Add support for deleting images uploaded unsing the Image Cropper property editor

This pull request adds support for deleting images uploaded using the ImageCropper property editor, both when emptying the Recycle Bin and when deleting individual items from the Recycle Bin.
I'm not particularly fond of special-casing property editors like this. It gets messy very quickly. Longer term the real solution would probably be for a property editor to somehow (perhaps via an interface?) indicate, that it has related files to delete.
But, for now I think it is better to have it this way than the current situation, which can fill up servers with old, unused images that are hard to clean up.