force sinknode global code execution

using a similar approach as with vmem we are now forcing the global
function execution of sinknode handles such that a derived class doesn't
need to manually call the sinknode's implementation.

instead the sinknode's public add/connect/disconnect functions are now
non-virtual and host the global code for node configuration and then
call protected virtual handles that can be reimplemented in the derived
class.

so, executionwise we now always have
SN::connecToMediaObject
-> VW::handleConnectToMediaObject
and
-> VW::handleDisconnectFromMediaObject
SN::disconnectFromMediaObject

any future sinknode implementation will need to drive the handles rather
than the public functions and they get called indirectly through the
public function. add/connect gets called after global setup and, whereas
disconnect is called before global teardown.