NativeEditor.ts and InteractiveWindow.ts could have their 'execution' logic split off

[rchiodo]

The 'execution' of a notebook is more than just calling requestExecute for a cell. It also involves posting results, ensuring a notebook is created and a bunch of other things.

It would be nice to make this more generic (like a service tied to a specific view) that would allow execution to be separated from the view itself. It modifies the view, but it doesn't exist on the view.