[Ur] Access to event object on onclick/et al

[Edward Z. Yang]

I'd like to implement different behavior in the case that the shift-key is
pressed down and a user clicks on an element, as opposed when nothing is
pressed down.  It is well known that event objects have a shiftKey attribute
which contains precisely this information.  However, Ur/Web event handlers
have no access to the event object.  To add insult to injury, there isn't
any way to FFI access to the event object either.

Unfortunately, I don't know how to add this without totally breaking
backwards comaptibility.

Edward