Bug 966727 - Toolbar created by sdk/ui should not be customizable

WIP, Unit Test is missing.

@gozala, I was trying to use the `customization` input we created in order to listen to the customization start / end once; but I wasn't able to make it working, I'm probably missing some crucial part:

```js
  const customization = new CustomizationInput();
  const { start, stop } = customization;

  once(customization, "*", message => console.log(message));
  start(customization);

  // this is toggle the customization mode.
  gCustomizeMode.toggle();
```

But only the `start` event is emitted, no `data`.