Add a menu function to replace the elevation value in waypoints for a selection of waypoints.

Issue #346 resolved
kiozen created an issue

Sometimes wayoint collections do not have the elevation value set or it's not very precise etc. In that case it would be nice to replace all values by the loaded DEM data. So far this is just possible in the waypoint's edit dialog for a single waypoint. This is not very convenient for several waypoints.