FlightGear - Flight Simulator FGAddon

* Dialogs/config.xml: remove multiple property-assign bindings and replace with
a single call to...
* Nasal/mosquito.nas (select_drop_tanks): new function.
(set_drop_tank_capacity): new function; adjust the capacity of drop tanks,
fill them and make them selected.
(main_loop): delete, redundant with the merlin engine automatic management in
merlin.nas.
* Models/stores/loads.xml: show the 200 imp gal drop tanks when selected.
* Nasal/failure.nas (gear3): delete; tail wheel is never steerable.
* mosquito-common.xml (consumables/fuel): add drop tanks.
(limits/mass-and-balance/maximum-takeoff-mass-lbs): new.
(help): explain drop tanks.
* mosquito-yasim.xml: add drop tanks with initial capacity of zero.