[css3-ui] grab & grabbing cursor (issue 26)

Issue 26 of css3-ui (https://wiki.csswg.org/spec/css3-ui#issue-26) states that we resolved to add the ‘grab’ and ‘grabbing’ values to the cursor property, but this had not yet been reflected into the spec. 

Here’s a patch attached to reflect that into the spec.

Note that this already has interoperable (and unprefixed) implementations in multiple browsers (tested Gecko, Blink, Webkit).

 - Florian