Make output widgets specific to widget manager with private rendermime reference

[jasongrout]

Output widgets depend very much on the widget manager. Right now, this forces the widget manager to provide interfaces for things like the document rendermime and the kernel context - things we generally don't want widgets to rely on because they are very specific to the widget manager.

It may be cleaner to special-case the output widget, so that it has a private reference to the rendermime and kernel context, but not expose those things to general widgets.

[jasongrout]

See also #1781

[pbugnion]

That sounds like a great idea. Having to reference the rendermime in the widget manager always seemed odd.