_gtkButtonScaleSizeSet sets the icon size to a
ButtonScale object.
Prototype : fun [ObjGtkButton I] ObjGtkButton
ObjGtkButton : a ButtonScale object.
I : the icon size, one of the following value :
ICON_SIZE_MENU, ICON_SIZE_SMALL_TOOLBAR, ICON_SIZE_LARGE_TOOLBAR,
ICON_SIZE_BUTTON (default), ICON_SIZE_DND or ICON_SIZE_DIALOG
Return ObjGtkButton : the same object,
or nil if an error occurs.