feat: support routing messages as custom events

[leostera]

Lets you send messages to the application process and receive them as custom events. This is needed to get data into the TUI from other parts of the system that weren't started from within the TUI.

Some use-cases:

• If the TUI needs to send itself custom events it can do that directly with send (self ()) The_message or through timers with Timer.send_after/send_interval
• If the TUI is just one more Riot App, then other Riot apps can talk to it.