Filtering for relevant configurationchange events

[eladalon1983]

There is the potential for many different configuration changes. What if an application is only interested in some? For example, we see this demo code by François, which currently reads:

function configurationChange(event) {
  const settings = event.target.getSettings();
  if ("backgroundBlur" in settings) {
    log(`Background blur changed to ${settings.backgroundBlur ? "ON" : "OFF"}`);
  }
}

This assumes that the only possible event is to blur, or else the following would be possible:

Background blur changed to OFF Background blur changed to OFF Background blur changed to OFF

A likely future modification of the code would be:

function configurationChange(event) {
  if (event.whatChanged != "blur") {
    return;
  }
  const settings = event.target.getSettings();
  if ("backgroundBlur" in settings) {
    log(`Background blur changed to ${settings.backgroundBlur ? "ON" : "OFF"}`);
  }
}

However, depending on what changes and how often, this could mean that the event handler is invoked in vain 99% of the time, and possible multiple times per second, needlessly wasting CPU on processing the event by JS code.

I think a more reasonable shape for the API, would be to expose a subscribe-method.

track.subscribeToConfigChanges(eventHandler, ["blur", "xxx", "yyy", "zzz"]);

Wdyt?

(Some prior art here is in w3c/mediacapture-screen-share#80. I propose my change as incremental progress over it. CC @beaufortfrancois, @guidou, @eehakkin and @youennf)