This PR takes a different approach to input handling.

Rather than returning the optional input handler, focus handle pair from
the element trait, we instead allow you to register an input handler
imperatively on the window context with `WindowContext::handle_input`.
You pass a focus handle reference and any implementer of
`PlatformInputHandler`. There's an `ElementInputHandler<V>` that
implements `PlatformWindowHandler` so long as `V` implements
`InputHandler`.

Release Notes:

- N/A