Progress & issue labeling on GitHub

With the recent PRs, we've brought the spec up to date with resolutions
from TPAC, with the exception of one issue [1].  This addresses the
horizontal feedback we've gotten thus far from other W3C groups.

As well, we've resolved a number of outstanding structural and consistency
issues with object and algorithms definitions.

I went through and tagged remaining items that require spec changes and
have some consensus in GitHub with the "action" label [2].  All
participants: feel free to submit PRs to move these forward.

The good news is that once the "action" issues are resolved, we are looking
at a handful of remaining spec issues.   I still think there's room for
improvement in the prose and flow of the spec, but overall we've come a
long way since TPAC.  Great work everyone!

m, your editor

[1] https://github.com/w3c/presentation-api/issues/99
[2]
https://github.com/w3c/presentation-api/issues?q=is%3Aopen+is%3Aissue+label%3Aaction