Add an entry point node to pipelines v2

[bderenzi]

This is purely a UI/UX thing and doesn't need to be saved. We should have a node that is pre-populated on the canvas for the entry point. We should not be able to delete the entry point. Maybe it should auto-connect to the input side of the first thing we drag in? (seems convenient).

I think it'll just help the user see where things start.