pls add event_handler() to add, and remove event handlers.

kthxbai

Relates slightly to #244

hmm, could be all added at the same time. My main complain is that if I made so that you can attach and detach player events, then all built-in apps can be loaded by default with carpet settings and don't cause any checks unless triggered, if you know what I mean.

done

You forgot to close #244 too