Accessor of the slot label of the gtk-button class.
The generic function gtk-button-label fetches the text from the label of the button, as set by the generic function (setf gtk-button-label).
If the label text has not been set the return value will be nil. This will be the case if you create an empty button with the function gtk-button-new to use as a container.
The generic function (setf gtk-button-label) sets the text of the label of the button to label.
This text is also used to select the stock item if the generic function gtk-button-use-stock is used.
This will also clear any previously set labels.