Merge Stable to master. Fix up conflicts

Cherry pick from master

More changes to ArduPilot manifest lookup

Cherry pick from master

Fixed typo in TerrainAirMapQuery::_parseCarpetData

Start Mission: Better error message

Add EGL to gstreamer dependecies

Flight mode fix, cherry picked from master

Plan: Default for terrain display is visible

Add note about ADSB server settings

Backported fixes

Better error message

Plan: CONDITION_GATE command support

Add note about ADSB server settings

Switch VideoReceiver to new API (big refactoring)

Plan: Correct handling of terrain ready when loading back from file

Correct handling of terrain ready when loading back from file

More changes to ArduPilot manifest lookup

Unit Test for Survey command sequence generation

Major rewrite of Survey generation for simplicity. Also includes support for CONDITION_GATE.

Temporary use setting for using CONDITION_GATE in Patterns

fixed preflight sensors check

Fixed 'if' typo in VideoReceiver::_makeSource method

Plan: Much better error reporting for mission transfer errors

Unit test for mission transfer error string checking

Much better error reporting for Mission transfer errors

Parallels build fix

Plan: Add support for takeoff item required