Emit picking event streams (bevyengine#16105)

# Objective

In `bevy_mod_picking` events are accessible through event listeners or
`EventReader`s. When I replaced event listeners with observers, I
removed the `EventReader` for simplicity. This adds it back.

## Solution

All picking events are now properly registered, and can be accessed
through `EventReader<Pointer<E>>`. `Pointer` now tracks the entity the
event targeted initially, and this can also be helpful in observers
(which don't currently do this).

## Testing

The picking examples run fine. This shouldn't really change anything.

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Co-authored-by: Aevyrie <aevyrie@gmail.com>