convert image attach selection box to only show unattached existing images

Now that we have #792444: UI to change order of attached images, the select box duplicates part of the draggable table's functionality: to remove a currently attached image you can either:

- tick 'remove' in the table
- unselect it in the select box

Given that select boxes are not very good UI anyway, we should change this to just being for attaching further images: it should just show image nodes on the system that are not currently attached.