Make comm manager (mostly) independent of InteractiveShell

This makes it possible to use comms from wrapper kernels, without instantiating the full IPython shell machinery.

The one remaining place where we need a reference to shell is to fire pre_execute and post_execute hooks (which are needed to get mpl figures right). This is a pure IPythonism, that it should be safe to ignore if shell is not set.

Ping @dsblank - you were asking about this in #6314.
I'm not sure. If a Widget can exist at all in a closed state, then I would say yes, it should be allowed to be instantiated without a CommManager present.