General event relay for xframe mode

[julianjelfs]

A couple of people have asked to be notified of particular events that occur within OC when they are hosting OC in an iframe. Rather than deal with each of these in an ad hoc way, it would be nice to design a generally extensible mechanism to relay significant events within OC to the iframe host so that we can easily add new events without changing all of the infrastructure that makes the integration work.