Some API changes:

 - I changed the second param of NotebookOutputRenderer.render to a
   `NotebookRenderRequest` since we were up to four parameters already.
 - The request contains a unique `outputId`, which is generated when
   output is first created or restored (the ID is not stable if a
	 notebook is closed and reopened)
 - I changed the renderer events to `onDidCreateOutput` and
   `onWillDestroyOutput` since they're for outputs, not cells.
 - Additionally, they're called with objects that contain the output
   ID to render.