Add ability for frontend to trigger events in the script

[sohkai]

It'd be great if there was a way for the frontend to send arbitrary events to the script, and define logic in the script to handle it.

It could look like app.trigger('<event name>', <event params...>) from the frontend, and @aragon/client's app.store() could receive these as-is.

See #313 (comment) for an description of how this could potentially be done.

[2color]

That's an interesting idea. Can you give an example of a use-case?

[sohkai]

A few come to mind:

• Fetching more data (if, in the future, reducing all of the state takes too long)
• Triggering an external API call that is better handled as part of the webworker
• Adding data that the script wouldn't otherwise know about from pure contract events (e.g. adding tracked tokens, Finance: add tracked token (without manual deposit) client#624)
• Optimistic UI for actions (multiple ways to do this, but "triggering" an "optimistic" event in the script could be one way)