Signals: events without code execution

[miraclx]

Permit peers to directly message themselves without needing to affect the state.

Enables things like informing themselves about activities like cursor position, online status, etc.

The current model for event propagation is "call a mutable method on your node, the method should emit an event".

This carries some cost since it has to load up the application only to perform such a trivial action as emitting an event.

We need to figure out how to prevent spamming other nodes. But that's not an an issue exclusive to this model.

Considering #188, I'd expect the execution cost to not be as significant. But raises the question of "signalling while other executions are taking place" which leads to #190. But a protocol-level feature for this won't need to contend with these big pieces.

But then we have to think about the model for representing these signals, and how to encode them in the app schema #270.

So it may not be worth it, but worth thinking about. Hence, this issue.