[css3-ui] using <image> instead of <uri> in cursor (Issue 44)

Raised in issue 44: https://wiki.csswg.org/spec/css3-ui#issue44

currently, the cursor property takes its images from a list of <uri>. It should use a list of <image> instead, to get access to the various functional notations defined for this value type. This is backward compatible as <uri> is a valid value of <image>.

Among other things (gradients, image fragments, crossfade...), this gives access to image-set(), which is needed to have high resolution cursor on “retina” displays.

Proposed patch attached.

 - Florian