Allow changing bus sync handler and appsink/src callbacks when running with 1.16.3 or newer
Previously it was not thread-safe to change them and could lead to crashes but with 1.16.3 it is now.
Unsetting the bus sync handler before 1.16.3 will have no effect at all, setting a new bus sync handler or appsink/src callbacks will panic.
This partially reverts 2f88dc65