Improve bevy_input_focus

[eckz]

Objective

I was curious to use the newly created bevy_input_focus, but I found some issues with it

• It was only implementing traits for World.
• Lack of tests
• is_focus_within logic was incorrect.

Solution

This PR includes some improvements to the bevy_input_focus crate:

• Add new IsFocusedHelper that doesn't require access to &World. It implements IsFocused
• Remove IsFocused impl for DeferredWorld. Since it already implements Deref<Target=World> it was just duplication of code.
• impl SetInputFocus for Commands. There was no way to use SetFocusCommand directly. This allows it.
• The is_focus_within logic has been fixed to check descendants. Previously it was checking if any of the ancestors had focus which is not correct according to the documentation.
• Added a bunch of unit tests to verify the logic of the crate.

Testing

• Did you test these changes? If so, how? Yes, running newly added unit tests.

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[viridia]

You learn something new every day :)

[viridia]

BTW, the prototype for tab navigation is here: https://github.com/viridia/thorium_ui/blob/main/crates/thorium_ui_headless/src/tab_navigation.rs

[alice-i-cecile]

Very nice, thanks :)