This stores the last request and resends it after a timeout, with a max
of 6 retries.
It also checks for old incoming packets and drops them.

Previously, a completed file upload lead to this assert:
    Q_ASSERT(!_ackTimer.isActive());
because _closeUploadSeesion() was called twice, thus activating the timer
twice.

* Some code moved to Vehicle

Moving source files into Vehicles as it is a more coherent place.

Forcing QGC to use Mavlink V2 if vehicle supports it.

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

This reverts commit ae976049ca7b4818616e6b8be07a028fc7dfc26f.