Adding interaction states for headless widgets. (ui only)

[viridia]

Part of #19236

This PR contains struct definitions for the low-level interaction states used by the headless widgets.

Testing

Tests will be in subsequent PRs. This PR contains only the basic data structures and hooks.

[viridia]

@alice-i-cecile @cart This PR contains only the bevy_ui changes, not the changes to bevy_picking which are still in the old PR.

[viridia]

Also please note the release notes are incomplete, and will be revised in follow-on PRs.

[alice-i-cecile]

Can you explain why you think we should use immutable components for the accessibility updates, rather than ordinary systems?

[viridia]

Can you explain why you think we should use immutable components for the accessibility updates, rather than ordinary systems?

I don't feel strongly about this, but my reasoning goes something like this:

• Reducing latency is good.
• Component hooks have very low latency. Using ordinary systems may entail a higher latency depending on what order they are run in the schedule. We can't necessarily predict where in the schedule the user will decide to mutate checked and disabled states, so we don't know where in the schedule is the optimal place to sync the accessibility updates.
• Using component hooks implies immutable components, because of the difficulty of detecting mutations.

All that being said, there are some advantages to using mutable components here: it makes the observers for input events simpler, since we can just mutate the states directly and don't have to resort to commands.

It's a trade-off, and I am not expert enough to know which option is better.

[alice-i-cecile]

Hmm. This departs from our designs elsewhere, but it's also a new set of tools. I really like being able to quickly respond to "a button was pressed" using the immutable components trick without having to iterate the whole set, and I would also very much like to avoid adding more callsites for RemovedComponents.

I'm alright with this design, but it should be swapped to use global observers added in the UiPlugin, not hooks. Hooks are intended for "true invariants" and cannot be overridden or disabled, and will be harder to configure and introspect in the future.