_gtkLabelAngleSet defines the angle of rotation in a Label
object. An angle of 90 reads from bottom to top, an angle of 270, from top to bottom, like trigonometric.
Prototype : fun [ObjGtkLabel I] ObjGtkLabel
ObjGtkLabel : the Label object.
I : the angle, in degrees (0 .. 360), measured counterclockwise.
Return ObjGtkLabel : the same Label object
or nil if an error occurs.