inject ember_debug in the target tab using webconsoleActor.evaluateJS (before DOMContentLoaded/DOMReady events)

- renamed to _onTargetTabLoad into _onDocumentElementInserted, attached to 'document-element-inserted' using
  Firefox 'observer-service' (needed to inject ember_debug before any page script run)
- use webconsoleActor.evaluateJS to evaluate ember_debug in the target tab
- queue messages if devtool panel is not yet attached
- reduced complexity in the target tab content-script injected
- use documentElement.dataset.emberExtension instead of the document.body element,
  because on Firefox when 'document-element-inserted' is called, the document.body element
  could be yet undefined.

If the content-script is executed before DOMContentLoaded, on Firefox document.body is not yet defined.