Support xlink:href in gradients

This is a relatively self-contained feature. Current (trunk) support for gradients ignores completely the xlink:href attribute, which means a good deal of content cannot be displayed in Design mode.

This feature would ensure that Carve's design mode follows xlink:href URLs to their associated gradients and accumulating gradient properties and stops along the way. This feature also needs to make sure that circular references are disallowed.