add Event messaging guidelines and requirements to the WAVE Content Specification

[johnsim]

A considerable amount of work has been done on a WAVE messaging white paper. General consensus in the November 7th conference call is that this should be a specification. Some relevant comments from @haudiobe include:

1. event processing model for client (event subscription, dispatch modes and timing model, dispatch API) - this work belongs in the DPCTF.
2. HATF ensure the DPCTF generic model maps to HTML5 APIs (note: dispatch API under discussion in the incubator task force, soon to be part of the media WG, and perhaps based on DataCue).
3. Content requirements - event stream rules, event message placing rules, special event message schemes. This work would be in the CSTF.
4. MPEG is currently fixing some details on Events

Based on this CSTF could add a event messaging section to the 2019 Content Spec, which includes normative language on event message placement, as well as informative guidelines, using the existing white paper as a starting point.