Make mocking picking events for bevy_ui easier

[alice-i-cecile]

Objective

As discussed in #17267, we'd like to unify input-focus driven widget activations (e.g. press enter to click the focused button) with mouse-driven picking interactions by going through the bevy_picking infrastructure.

While you can fake this with observer triggers, it is:

• inaccurate: you're feeding in garbage information
• extremely verbose
• rather arcane

Solution

As @aevyrie points out, we have access to everything we need to send realistic pointer events! Doing this correctly requires rather a lot of information. To make users lives easier, I've introduced a system param with all of the required information, which simulates pointer events at the center of the UI node specified.

The directional_navigation example has been updated to demonstrate / test this change.

I've also removed the Component impl for bevy_picking::Location as this is unused and a serious footgun: I wasted quite a bit of time trying to debug what had gone wrong. See #17390.

Discussion

• Some small improvements to this general area of the code have been split out into Small improvements for directional navigation #17395. They'd be nice here, but I wanted to reduce complexity here.
• While PointerId::Custom exists, it is challenging to both generate a UUID and to match against it if you want to respond differently if the Pointer event is from a focused element. While I could have used sentinel values here, I think it's more idiomatic and nicer to use if we just add an enum arm for a common use.
• We should probably rename bevy_ui/picking_backend.rs, but I've opted to defer that to make the diff easier to read.
• This was originally a custom command, but has been refactored to a SystemParam for flexibility, easier error handling and performance.
• As follow-up, we could add some simple helper methods to the SystemParam to make certain actions even easier.
• In theory, other backends could do the same thing! The logic required is slightly specialized, so I stuck to just UI for now.

Testing

I've tested this with the directional_navigation example. While I hope that this works properly with more exotic windowing / rendering target setups (viewports, portals, multiple windows), we're not equipped to do that effectively right now.

Migration Guide

bevy_picking::Location is no longer a component. This was a bug: you wanted PickingLocation.

PointerId has a new variant, PointerId::Focus that will need to be exhaustively matched against.

[viridia]

A concern I have is that the motivation, use case, and overall theory of operation won't be apparent to people who are looking at the docs for Pointer. I would like to see a more detailed note, and possibly link to the emulated pointer documentation.