Set or get if a click on a button give the focus to this button.
Définit ou récupère l'état du focus lors d'un clic sur un bouton.
_button_ : ObjGtk : any button already created.
_value_ : I : If _value_ is 1 (or not nul value), the focus is given to the button when it clicked. If _value_ is nul, the focus is inchanged when the button is clicked. The default value is 1.
If _value_ is 'nil', the function give the current state.
Return : I : _value_ is _value_ is not nil, otherwise the current state (0 or 1). Return nil if _button_ is not a valid object.