[mediacapture-handle] CaptureHandleChangeEvent should just be Event (#50)

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-handle:

== CaptureHandleChangeEvent should just be Event ==
The [doc](https://w3c.github.io/mediacapture-handle/identity/index.html#capturing-side) says _"Capturing applications who are permitted to [observe](https://w3c.github.io/mediacapture-handle/identity/index.html#dfn-observable) a track's [CaptureHandle](https://w3c.github.io/mediacapture-handle/identity/index.html#dom-capturehandle) have two ways of reading it."_ 

This is one too many. The second way:
```js
let handle;
track.oncapturehandlechange = event => handle = event.captureHandle();
```
...seems entirely redundant, since this should suffice:
```js
let handle;
track.oncapturehandlechange = ({target}) => handle = target.getCaptureHandle();
```
I believe the common pattern in situations like this is to just fire a vanilla `Event`.

Please view or discuss this issue at https://github.com/w3c/mediacapture-handle/issues/50 using your GitHub account


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