[DOM3Events] InputDevice API sketch

In our latest discussion of how best to identify mouse events derived from
touch events, I proposed a 'sourceDevice' property
<https://lists.w3.org/Archives/Public/www-dom/2015JanMar/0052.html>.  Domenic
asked <https://lists.w3.org/Archives/Public/www-dom/2015JanMar/0052.html>
for a rough sketch of what such an InputDevice API might grow to become.
Here
<https://docs.google.com/a/chromium.org/document/d/1WLadG2dn4vlCewOmUtUEoRsThiptC7Ox28CRmYUn8Uw/edit#>
is my first attempt at such a sketch, including some detailed references to
similar APIs in other platforms.  Any thoughts?

Note that at the moment I'm primarily interested in standardizing and
implementing the 'firesTouchEvents' bit.  However if it makes more sense
for coherency, I'd also support adding (and implementing in chromium) a few
of the other non-controversial pieces.

Rick