Add advice on best practices for handling event propagation

[alice-i-cecile]

Objective

@viridia mentioned this advice off-hand.

It was good advice (and used in our UI crates), but was never written down.

Solution

Write down and cross-link the advice.

[viridia]

One more nuance: if a handler, handling an event, wants to trigger additional behaviors further up the hierarchy, the best practice is to define a new event type for this, rather than re-using the event.

[viridia]

This means that in a complex, multi-layered hierarchy comprised of nested, reusable modules, the events will evolve and transmute as they travel upwards, starting out as low-level machine signals (like "key press") and eventually becoming high-level, semantic, meaning-laden events such as "save document" or "pause animation".