FlightGear - Flight Simulator FGAddon

* Models/submodels.xml: new submodels for drop tanks.
* Nasal/mosquito.nas (jettison_drop_tanks): new.
* mosquito-common.xml (keyboard): Ctrl+D to drop tanks;
call the above in the binding.

Additional bug fixes and cleanups:

* Nasal/mosquito.nas (capture_heading_listener,
capture_altitude_listener): new;
capture heading and altitude when user engages autopilot with
Ctrl+H or Ctrl+A respectively.
(fuel_flow_timer): new, adapted from the Lockheed1049h.
* mosquito-common.xml (keyboard): Swap F and Shift+F (cooler
flaps) for consistency with the Lockheed1049h.
* Nasal/merlin.nas: rename merlin66 to just merlin.
* Models/Instruments/vsi40-gb/vsi40-gb.xml: bug fix: correct the
rotation of the needle.
* Models/loads.xml: delete, unneeded (Models/stores/loads.xml instead).