[css3-ui] considering adding cursor:grab; cursor:grabbing per Gecko/WebKit interop

This test case demonstrates there is now prefixed value support for
cursor:grab; cursor:grabbing in both Gecko/WebKit interop:

http://frankyan.com/labs/cursor/

Before I saw the demonstration of interop, I was considering these for
CSS4-UI: http://wiki.csswg.org/spec/css4-ui#new-cursor-values

However since we have interop they should land in a spec that will go
back to CR soon so we can unprefix them ASAP.

As such I'd like to add these to the definition of 'cursor' in CSS3-UI
and issue another last call (since we can't add features between an LC
and a CR, right?).

Any objections?

Tantek