Reconsider design of `WindowTraversal` to decouple `input_focus`

[alice-i-cecile]

Originally posted by @ickshonpe in #19736 (comment):

I really don't like that part of the design of WindowTraversal then, but I can see that it's out of scope to complain about it here. Feels like probably the input focus module shouldn't need to have any knowledge of windows at all.

Given that there are several places in Bevy where a UI node wants to know what window it's on, I wonder if it makes sense to propagate the window ref downward in the hierarchy (now that we have propagation).

The UI already does window entity propagation through the ComputedNodeTarget component. Maybe the propagation system needs to be replaced using the new features though.