Edits to SVG-AAM

I've pushed a number of clarifications and clean-up edits to the SVG-AAM to
a branch of the same name, and also made the defaults in the role mapping
table use the new graphics roles.

Pull request for discussion: https://github.com/w3c/aria/pull/373
Compiled version in RawGit:
https://rawgit.com/w3c/aria/svg-aam/svg-aam/svg-aam.html

I've also added updates to relevant issues in GitHub.  I think this should
address most of Joanie's concerns, but I'd appreciate her confirmation on
that!

I still need to make corresponding edits in SVG2, including the alternative
approach to presenting default and allowed roles that I proposed at today's
teleconference.  I'll send a follow-up email when those are ready.

Best,
Amelia