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.
See g_signal_new for details on allowed signal names.
| itype | the type that the signal operates on. |
| name | the signal's name. |
| the signal's identifying number, or 0 if no signal was found. |