Split observer events

[tim-blackbird]

Objective

• Split the two types of observer usages - trigger and trigger_targets - by trait

Solution

Check out the diff of the release notes.

Notes

• It's not possible to derive BroadcastEvent and EntityEvent together, one must be implemented manually if you want both. This is probably okay as having both implemented is somewhat undesirable.
• Planned follow-up to this PR is to rename Event to ObserverEvent.

[alice-i-cecile]

What to do with the SceneInstanceReady event which is the only event in Bevy to use both trigger and trigger_targets.

We can replace the untargeted form with a buffered event IMO.

[alice-i-cecile]

Really excellent docs!

[alice-i-cecile]

Very nice. I'm pleased with how simple the diff is. I'm increasingly fond of this as the final form: this should give us 99% of the type-safety benefits without any complexity around mutually exclusive traits or a On variant for broadcast events.

Well, at least after Event -> ObserverEvent is done :)

[alice-i-cecile]

I spent some time today writing up a focused little design doc for the ultimate split here: https://hackmd.io/@alice-i-cecile/SyuIWJPLxg

I don't particularly care whether we do it in a single PR or across multiple, as long as we converge to our ideal form before we ship 0.17.