_finalize methods need information which is not available if all you
have is a mavlink_message_t

Instead us a Toolbox concept which hangs off of QGCApplication

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

- Download working again
- Fixed lots of bad error handling

writeFileAccumulator contents cleared on errors, nack or ack timeout
memcpy used to transfer data to request message buffer

Plus a large number of other changes to allow for orderly shutdown of
objects without crashes or asserts. This is need for unit tests to
create/delete global state around every test.

Otherwise we can end up generating strange CRCs.

- Also corrected system/component id usage
- Switched to new Qt5 connect api

Also restructured signals to be more command-centric as opposed to
ui-centric.

Added:
- sequence numbers
- better filtering for image encapsulated data messages versus FTP
- directory list now contains file sizes
- open command now returns file size in data