If the label has been set so that it has an mnemonic key, the label can be associated with a widget that is the target of the mnemonic. Return this current widget.
Retourne le widget cible d'un mnemonic du label, si ledit label a été définit avec un mnemonic.
_label_ : ObjGtk : any label already created.
Return : ObjGtk : the widget or nil if error or _label_ didn't set to a mnemonic or no widget associated.