Cleanups in QObject bindings

* Rename v8object to jsWrapper in QQmlData
* Rename v8objectid to jsEngineId, as that's the identifier of the engine
  that currently owns the primary JS wrapper

This is in preparation for moving newObject away from QV8QObjectWrapper

Change-Id: I6432365e849d159600e22f09e7e2ab2ae2117db6
Reviewed-by: Lars Knoll <lars.knoll@digia.com>