This change allows one to mark items from multiple child-resources displayed on one page and add these to the clipboard. Use the same logic for bluk-delete and bulk adding to the clipboard.
Make :folder_id available as a package instance variable immediately, in a way that if the folder instantiation needs e.g. to fetch a parameter, the folder_id will be there to determine the right parameter page, which otherwise would be looked up e.g. in the site-wide pages and cached with wrong values for the cache lifetime