Changing mission altitude reference doesn't change WP altitudes

#### Issue details
After mission created or during the process and WP input in Flight Plane view you may want to change altitude reference for the mission from Absolute to Relative or vice versa. After changing altitude reference all altitudes in WP doesn't change it's value regarding new altitude reference and home altitude, It may cause an error when you create a mission then write it, switch from Rel to Abs for some reason and then read it from AP to MP, You'll have different altitudes and if you'll write it back to AP it may cause crash.

It's better to convert altitudes at reference change using current home elevation.
#### Version
MP 1.3.55

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

Start in the Flight Plane view's mission and waypoint altitude-reference handling, which is the entry point named in the issue. Reproduce switching between Absolute and Relative references before and after mission creation, using the current home elevation. Done means waypoint altitudes are converted consistently and missions can be read from and written back to the autopilot without differing values or causing a crash.

Written by the indexing model from the issue text.