[css-ui] cursor property should allow multiple non-url values

[lolbinarycat]

Currently, the cursor property is defined as any number of url-based cursors, followed by exactly one predefined value.

This design assumes that all graphical environments will have all of these cursors defined, which is quite an optimistic assumption.

Which cursors are available ultimately depends either on the underlying platform cursor theme, or which cursors have a fallback embedded in the user agent.

I believe that cursor should work equivalently to font-family, where multiple predefined cursor styles can be intermixed, and url cursors could potentially even appear after url cursors.