Edit multiple altitudes at once in flight plan

After laying down over a hundred waypoints, I decided I needed to lift them all up by a certain amount. It would be nice to have a function in MP that allows the editing of altitudes in a batch rather than one at a time.

No file, test, or entry point is named in the issue. Start by tracing Mission Planner's flight-plan waypoint altitude editing and selection flow, then define the intended batch behavior and verify that users can adjust the altitudes of many waypoints instead of editing them individually.

Written by the indexing model from the issue text.