- select active vehicle
- arm/disarm
- select flight mode

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

Also many LinkManager API changes to further isolate all link
management inside LinkManager.

Moving methods from MainWindow to QGCApplication so that more things
can run without a window. Helpful for testing.

So no need for connect window close test here