Add scroll height indicators to Flickable

Direct connect to PX4 Flow shows up as generic

Windows OS doesn’t disconnect port when you pull plug on USB. This
caused all sorts of problems with previous logic. New logic handles
this correctly.

Plus lots of other tweaks to ui for visuals

- 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