Overhaul how transitions are stored in FlightPlan XML, and how
they’re exposed to Nasal. Simplify the Nasal access by making
‘sid_trans’ and ‘star_trans’ writeable.
Extend the unit-tests a lot to cover this, both from C++ and also
from Nasal
As part of this, overhaul the ownership of FlightPlan delegate
factories, to make it safer (use ref-counting of the factories,
and allow the factory to customise delegate clean-up behaviour)
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