app, ui, server

[jonthegeek]

De-cute the names a bit. And, now that I grok that we can change the server (by wrapping all servers in a wider server function that decides which one to call), I think the most common use case will be wrapping an entire ui + server, and only occasionally will people want to separately wrap UI and server. Note that under the hood they're still independent somewhat, because ui is decided using request, while server is via session$request.

• Keep set_scene() and change_scene(), although possibly with a deprecation warning.
• Those will just be aliases for set_ui() and change_ui().
• Add set_app() and change_app()
• Add set_server() and change_server()

Split this into multiple tickets if necessary, but I suspect I can do most/all of it at once cleanly.