Add AttachedItemsMixin
Add AttachedItemsMixin

Adds 'attached_items' property, which allows structured attachment information to
be easily obtained from an object.

mvidalgarcia/indico,DirkHoffmann/indico,DirkHoffmann/indico,mvidalgarcia/indico,OmeGak/indico,OmeGak/indico,DirkHoffmann/indico,OmeGak/indico,DirkHoffmann/indico,ThiefMaster/indico,ThiefMaster/indico,ThiefMaster/indico,indico/indico,indico/indico,mic4ael/indico,pferreir/indico,mvidalgarcia/indico,OmeGak/indico,mvidalgarcia/indico,pferreir/indico,ThiefMaster/indico,indico/indico,mic4ael/indico,pferreir/indico,pferreir/indico,mic4ael/indico,mic4ael/indico,indico/indico