_gtkWidgetCBevent defines a callback to a Widgets object
when a signal goes be emitted. This reflex will be called either before this signal or after (see flag, below).
Prototype : fun [* fun [* u0 S I] u1 u0 I I] *
* : a Widgets object.
fun [* u0 S I] u1 : a callback. These arguments are :
* : the Widgets object (the emitter)
u0 : the user parameter
S : the GTK signal name (see below)
I : the type of the signal
u0 : a user parameter, at your convenience
I : a flag (one of these values):
CB_EVENT : before the signal
CB_EVENTAFTER : after the signal
I : the propagation flag : SIGNAL_NOPROPAGATE or SIGNAL_PROPAGATE.
Return * : the same Widgets object
or nil if an error occurs.