Display GPS Altitude

Could we add the GPS Altitude estimates to the Status page of Mission Planner so we can possibly use it for adjusting altitude offsets of the barometer over long flights if the GPS readings seem reasonable?

Discussion here: https://github.com/diydrones/ardupilot/issues/2506#issuecomment-118562406

Thank you

Start at Mission Planner’s Status page and inspect how existing GPS and barometer readings are presented. The work is done when a GPS altitude estimate is displayed there for comparison with the barometer, with the usefulness of the reading considered as described in the issue.

Written by the indexing model from the issue text.