Since the switch to wrapping classes using PHP's C API, we now
internally need to be able to tell if a PHP object is of or derived
from a class that is wrapped by SWIG so we know if we can offset the
zend_object pointer to get to the swig_object_wrapper. If we try to
do this to an object which isn't wrapped by SWIG then we invoke C/C++
undefined behaviour (and typically get a segmentation fault).
This check is implemented by having a SWIG\wrapped empty interface which
we make all SWIG-wrapped classes implement simply so we can test for it
to detect such classes.
Fixes#2125
