Make it possible to stop apps programmatically

[erikbern]

Really just moving some code from the cli package into the main code.

However I realized it's a bit janky because ideally you'd want this to be an instance method, and not a class method. But that would require better ways to construct an App object directly. Which we don't. We should address this though!

Also cleaned up some synchronicity stuff.

[mwaskom]

But that would require better ways to construct an App object directly.

we have App.lookup now; do you mean something other than that?

[erikbern]

we have App.lookup now; do you mean something other than that?

Yeah the CLI lets you pass either an app name or an id – so we would need something to construct an app from an id.

Maybe I should just add that. It would be pretty similar to how lookup works.