DbViewer: to compare nodes in Constraints View that are not directly linked, Graph View should be open first, which is not obvious

Possible solution:
* Show a message box to tell the user to open the Graph View if they want to compare nodes not linked in Constraints View. Ask only once per session, and do it only when we detect that the Constraints View is open, odometry links exist, Graph view is not open and selected nodes are not linked.