We store a pointer to the registered property set on the Stylist because
that seems to be the cleanest way to give access to it to all the
functions that might need to cascade or compute typed custom properties.

This also lets us use the Stylist's existing mechanism for determining
we need to rebuild styles: if it last saw the registered property set
with an older generation than the current generation, we should do
things again.

This is handled by the same path in layout_thread::lib as the device
being dirtied or the stylesheets changing.

Also update the comment on Stylist::rebuild, which appears not to have
been updated when stylesheets_changed was replaced with
origins_to_rebuild.

Part 9 of a series of patches to implement the CSS Properties & Values API.