defvc components no longer declare the view-sigs that will be used.
calls to view-cursor within the component's render function now
automatically trigger the component to subscribe/unsubscribe to the
view-sigs being passed to any view-cursor calls.
to know when it requests should be sent to the server to subscribe
and when to unsubscribe (if multiple components subscribed to the same
view-sig, etc)
- includes some related refactorings
- getting a view cursor by view name (not sig) now is limited to a
view-sig lookup within the list of the component's own subscribed
view-sigs instead of the global subscribed view-sig list