Adds helper functions to set the links start and end position with global positions or get them as global positions.
Adds global start and end position for the navigation link to the 'link_reached' signal of NavigationAgent. That signal gets emitted when a navigation link waypoint is reached. Requires that 'owner' meta data is enabled on the NavigationAgent.
The navigation team has some large changes planned for navigation in the 4.x timeframe, so marking these nodes as experimental to give users a heads up that the API may change in breaking ways.
