Changing MP measurement units doesn't change altitude value for mission

#### Issue details
After you input WPs in Flight Plan view and then changed altitude units (meters to feet or vice versa) altitude values for already input WPs will not convert. It's better to convert those altitudes, to avoid the need to correct every WP manually. Also if you wrote the same WP in meters and then in feet it will result in different altitudes and may cause the crash.

#### Version
MP 1.3.55

#### Platform
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

Start in Mission Planner's Flight Plan view and trace how waypoint altitude values are stored and displayed when the measurement unit changes. Reproduce the meters-to-feet and feet-to-meters cases, then verify that existing waypoints convert consistently and that entering the same waypoint in either unit produces the same altitude.

Written by the indexing model from the issue text.