_gtkButtonCBclicked defines a callback to a Buttons
object when a "clicked" signal is emmitted.
Prototype : fun [ObjGtkButton fun [ObjGtkButton u0 S] u1 u0 I] ObjGtkButton
ObjGtkButton : a Buttons object.
fun [ObjGtkButton u0 S] u1 : the callback (like @function_name).
Arguments are : the Button object, the user parameter and the name of emitted signal (here "clicked").
u0 : a user parameter, at your convenience.
I : a flag : SIGNAL_PROPAGATE or SIGNAL_NOPROPAGATE.
Return ObjGtkButton : the same Buttons object
or nil if an error occurs.
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