Add "bevy_input_focus" crate.

[viridia]

Objective

Define a framework for handling keyboard focus and bubbled keyboard events, as discussed in #15374.

Solution

Introduces a new crate, bevy_input_focus. This crate provides:

• A resource for tracking which entity has keyboard focus.
• Methods for getting and setting keyboard focus.
• Event definitions for triggering bubble-able keyboard input events to the focused entity.
• A system for dispatching keyboard input events to the focused entity.

This crate does not provide any integration with UI widgets, or provide functions for
tab navigation or gamepad-based focus navigation, as those are typically application-specific.

Testing

Most of the code has been copied from a different project, one that has been well tested. However, most of what's in this module consists of type definitions, with relatively small amounts of executable code. That being said, I expect that there will be substantial bikeshedding on the design, and I would prefer to hold off writing tests until after things have settled.

I think that an example would be appropriate, however I'm waiting on a few other pending changes to Bevy before doing so. In particular, I can see a simple example with four buttons, with focus navigation between them, and which can be triggered by the keyboard.

@alice-i-cecile

[alice-i-cecile]

I really want this, and I like the fundamental design. At the start of the 0.16 cycle I'm going to come back to this, evaluate scope and then decide how to proceed. We may want a working group for all of the follow-up.

[viridia]

One thing that has not been established is the relationship between this and bevy_a11y. I had originally assumed that this would depend on bevy_a11y, but it sounds like you were thinking that it would be the other way around.

[alice-i-cecile]

Yeah, my expectation is that bevy_a11y is a consumer of the focus data produced here from user input, and so should depend on this crate.