This adds a register_if() method to the object registry, which allows for
conditional registration of the object class. It does, however, always
build the class properties and remotable methods, even if registration is
not performed. This is useful when it is desirable to control which
implementation of an object is used, or for testing purposes.
Change-Id: I0deb82744e7db659e8e75ce300b2c5417830f18d
5138b697b6