_gtkLabelSelection returns if the selection of a Label
object is not empty and gets the selected range of characters in the Label.
Of course, the Label must be selectable.
Prototype : fun [ObjGtkLabel] [I [I I]]
ObjGtkLabel : the Label object.
Return [I [I I]] : empty (0) or not empty (1),
and a tuple : the start and the end offsets
or nil if an error occurs.