Distinguish .target() and .target in Trigger<Pointer<E>>

[Zeophlite]

Objective

• .target() and .target on Trigger<Pointer<E>> refer to two separate entities
• Fixes Target::target and Target::target() return different entities #17112

Solution

• Describe the solution used to achieve the objective above.
• Added an example that demos picking in a heirachy

Testing

• Ran cargo build and cargo test --all-targets
• Ran directional_navigation example

[chescock]

I think #18710 is trying to tackle this same confusion, while also making the "original target" available for all events instead of just Pointer events. It's also proposing a different naming convention of target vs current_target. I don't know what the right answer is here, but it might help to combine the discussions!