Issue 18 Stage 3: make filter attributes into domain modules

I've completed the stage 3 proposal for making @audience, @platform, @product, and @otherprops into specializations, and integrated review feedback from Deb and Dawn.
This one is now ready for discussion at the TC: (See attached file: Issue18-stage3-specializeProfileAtts.pdf) If you want a refresher on the reasons for this change, the use cases and technical design are in the stage two proposal linked at the top of the document: https://lists.oasis-open.org/archives/dita/201802/msg00104.html Thanks, Robert D. Anderson DITA-OT lead and Co-editor DITA 1.3 specification Marketing Services Center E-mail:  11501 BURNET RD,, TX, 78758-3400, AUSTIN, USA Attachment: Issue18-stage3-specializeProfileAtts.pdf Description: Adobe PDF document