FlightGear - Flight Simulator FGAddon

* Decided that the model "PTB-260" is the closest match for the
drop tanks presented in the code, and adjusted some properties
according to that choice.
* Moving the tank models a little bit up, closer to the wing (it's a "slipper tank")
* Set the tank capacity to 260 l each
* Add the empty tank weight to the FDM
* Model each tank independently, as a preparation for future damage simulation
* FDM: Simulate "drag due to drop tanks" by two external forces
* FDM: Add "pitch due to drop tanks"
* Configuration menu can toggle drop tanks only when the aircraft is being parked
* Newly attached drop tanks are empty
* Replac "tanks/fastened" by "tanks/attached"
* Correct spelling droptank -> drop tank