_gtkButtonSwitchSet changes the state (on or off) of a ButtonSwitch
object.
The used GTK+ version should be 3.x or above, otherwise the object will not
be created and nil will be always returned.
Prototype : fun [ObjGtkButton I] ObjGtkButton
ObjGtkButton : a ButtonSwitch object.
I : the new state : on (1) or off (0, default).
Return ObjGtkButton : the same ButtonSwitch object
or nil if an error occurs.