Add option to move or copy between galleries

Sometimes we'd like to have the same image appear in multiple galleries and sometimes we will initially put the images in the wrong gallery. The way it works currently, we have to delete the image from the gallery, then upload it again into the new gallery - this also includes the issue of having to then redo the caption.

Since one of the great factors in the extension is that things can be effortlessly dragged around from local to site and reordering within a gallery, it seems a natural extension to allow dragging from one category to another or at least being able to "tag" images with the galleries they should be in or something similar.

Thanks