- MultiVehicleManager is a Qml friendly replacement for UASManager
- Vehicle is a replacement for both UAS and AutoPilotPlugin

Also fixed case where UASMessageHandler reported errors but walking
list turned up no errors

If Facts are referenced from FactPanelController, we need to reference
count the AutoPilotPlugin such that it doesn’t get destroyed until
after the FactPanelController is destroyed.

Otherwise they end up referencing deleted Facts on link disconnect

Was causing unnecessary read re-requests to get sent out

QGCUASParamManager and friends are replaced by FactSystem

More consistent with Qml usage pattern