lua : add a function to remove an image from the database

dt.database.delete(image)

or

image:delete()
lua : add a function to move an image

dt.database.move_image(film, image)
film:move_image(image)
image:move(film)

this moves the image file, the associated XMP and the XMP for
the duplicates to the directory pointed by the film
lua : add a function to copy an image

dt.database.copy_image(image,film):image
image:copy(film):image
film:copy(image):image