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
ace0114bdd