…19663)

# Objective

Getting access to the original target of an entity-event is really
helpful when working with bubbled / propagated events.

`bevy_picking` special-cases this, but users have requested this for all
sorts of bubbled events.

The existing naming convention was also very confusing. Fixes
#17112, but also see #18982.

## Solution

1. Rename `ObserverTrigger::target` -> `current_target`.
1. Store `original_target: Option<Entity>` in `ObserverTrigger`.
1. Wire it up so this field gets set correctly.
1. Remove the `target` field on the `Pointer` events from
`bevy_picking`.

Closes #18710, which attempted
the same thing. Thanks @emfax!

## Testing

I've modified an existing test to check that the entities returned
during event bubbling / propagation are correct.

## Notes to reviewers

It's a little weird / sad that you can no longer access this infromation
via the buffered events for `Pointer`. That said, you already couldn't
access any bubbled target. We should probably remove the `BufferedEvent`
form of `Pointer` to reduce confusion and overhead, but I didn't want to
do so here.

Observer events can be trivially converted into buffered events (write
an observer with an EventWriter), and I suspect that that is the better
migration if you want the controllable timing or performance
characteristics of buffered events for your specific use case.

## Future work

It would be nice to not store this data at all (and not expose any
methods) if propagation was disabled. That involves more trait
shuffling, and I don't think we should do it here for reviewability.

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Co-authored-by: Joona Aalto <[email protected]>