rulerView(_:shouldAdd:)
Requests permission for aRulerView to add aMarker, an NSRulerMarker being dragged onto the ruler by the user.
Declaration
func rulerView(_ ruler: NSRulerView, shouldAdd marker: NSRulerMarker) -> BoolDiscussion
If the client returns true the ruler view accepts the new marker and begins tracking its movement; if the client returns false the ruler view refuses the new marker.