New principle: use "user attention" to prevent abuse of/by certain APIs

[marcoscaceres]

This was brought up in our discussion of Screen Orientation APIs use of user attention by @bvandersloot-mozilla:
w3c/screen-orientation#262

In which:

If I understand correctly, these events only fire on visible, top-level windows. This seems like a great property to hold!
...
It would be interesting to be able to make this a recommendation to other APIs that issue events that may reveal user information, but are essential to a page working.

And relavant pull request:
w3c/screen-orientation#266

[jyasskin]

See also whatwg/html#6211 which drove the addition of "fully active descendant of a top-level traversable with user attention".