LinkInterface and Vehicle uses mutexes to synchronize writing bytes to the link instead of queued signals

App messages now use simple modal message dialogs. Vehicle messages continue to use non-modal popup.

Also fixed bug with ArduPilot guided waypoint write which would trash
existing mission.

* Resume index adjust to previous coordinate command
* Takeoff always added to resume mission
* Remove camera id checks which are no longer part of spec
* Simplified de-dup logic

Remove/comment off debug output pollution

This time to handle correctly updating plan/fly view when plan changes

Works around ArduPilot strangeness with empty missions end

Must memset structures before use.

Fixed a pile of bugs as well

Plus low battery announce changes