Changing the notification protocol for core_widgets.

[viridia]

Notifications now include the source entity. This is useful for callbacks that are responsible for more than one widget.

Part of #19236

This is an incremental change only: I have not altered the fundamental nature of callbacks, as this is still in discussion. The only change here is to include the source entity id with the notification.

The existing examples don't leverage this new field, but that will change when I work on the color sliders PR.

I have been careful not to use the word "events" in describing the notification message structs because they are not capital-E Events at this time. That may change depending on the outcome of discussions.

@alice-i-cecile

[alice-i-cecile]

Good, I suspect this will be useful regardless of the final direction we end up choosing.

[ickshonpe]

It always bothers me how fragile objects composed from multiple entities seem in Bevy.
Agree the source entities are necessary, and the input value wrapper types provide some extra context to the callbacks which might be helpful to users I guess.