Dragging an image into the stack canvas, then not saving leaves image file behind

[uliwitness]

If you drag an image file into the stack canvas window to add it as an icon and then don't save, we do not add an entry to the stack's media list for that image as requested. However, we do copy the file into the stack's bundle and don't delete it again. We don't really want to accumulate waste that way and should probably take note that we've copied the image already and want to delete it.