Informs the client that
a will add the new NSRulerMarker,
- macOS 10.0+
location is the marker’s tentative new location, expressed in the client view’s coordinate system. The value returned by the client view is actually used; the client can simply return
location unchanged or adjust it as needed. For example, it may snap the location to a grid. This message is sent repeatedly to the client as the user drags the marker.