Tutorial cards to explain UI interactions

[bergie]

We should look at how games like Angry Birds and Cut the Rope handle their onboarding, and show cards to assist new users in interacting with the system.

Some examples:

• When user creates an empty graph, we should show a card visualizing how to add a node from the component library
• After the node has been added, show how to interact with it (#53)
• When the first edge has been added, show how to get rid of it