notebook: animation, interactive

[switham]

Allow interactive animations and graphic games within a notebook.

Since we will be running the user's code in the server, this means besides streaming the drawing effects to the browser, simultaneously catching input events in the browser and streaming them to the server as HTTP requests.

Would either have to

• distinguish game events from other notebook interactions like interrupt, pause, edited a code block, etc., or
• have games go into full-screen mode, and, say, the browser back button is caught and escapes you from the game.