Given the name of the signal and the type of object it connects to, gets the signal's identifying integer.
Emitting the signal by number is somewhat faster than using the name each time.
Also tries the ancestors of the given type.
The type class passed as
itype must already have been instantiated (for example, using
class_ref) for this function to work, as signals are always installed during
See @new for details on allowed signal names.
the signal's name.
the type that the signal operates on.
the signal's identifying number, or 0 if no signal was found.