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.
Définit le widget cible d'un mnemonic du label, si ledit label a été définit avec un mnemonic.
_label_ : ObjGtk : any label already created.
_widget_ : ObjGtk : any widget already created or nil to unset it.
Return : I : 0 or nil if error (_label_ doesn't exist).