Drag and drop output ref

Spec: https://docs.google.com/a/dimagi.com/document/d/1TjJ3vUjH3Xc_dkYd8pKoErVFjcXkbsYlfClSlcJ9Ic0/edit#

@millerdev 
Ideally core would know nothing about itext since javaRosa (the place where itext lives) is a plugin. I know there are other places where this bleeds through, but hoping to reduce those occurrences rather than add new ones. Maybe it would be very hard or impossible to not reference itext here?