createWindow QML API for QQuickWebEngineView

This implements adoptNewWindow for QQuickWebEngineView.
The API is only intended to be used through QML to avoid delegating
the QQuickWebEngineViewHandle ownership through a signal parameter.

Another limitation of the implementation is currently to fail the
handle adoption unless it is done synchronously within the
adoptNewWindow call. To support this we would need to delay the call
to WebContentsAdapter::initialize which will leave the adapter
without a client when returning to the event loop and would require
putting null checks everywhere it is used.
So I would prefer to keep the API limited and avoid potential crashes.
If we want to support asynchronous Loader elements or QML files
fetched from the network in the future, the API should be able to
scale to the task once we've adjusted the implementation.

This also adds basic tabs support in the quicknanobrowser example.
The url property is now set imperatively to avoid overwriting the
adopted WebContentsAdapter's loading URL.

Change-Id: Iba5c5dc3ffa21045f356be131ca15c01b9aee7c8
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>