API: add waypoint to existing track

Over track recording UI you could click on "create waypoint" to include a point into the track. It would be nice to be able to create track waypoints over API.

Main advantage is the ability to display them in elevation diagram.

This could be use full to place all kind of helpful icons on the elevation diagram to make it easier to plan the trip.

• It would be nice to modify existing tracks
  
• modify current recording track