FlightGear - Flight Simulator FGAddon

* Nasal/bombable.nas: new, copied and adjusted from the MiG-15.
* Models/submodels.xml: add ammo for the four .303 machine guns and
four Hispano Mk II 20-mm cannon.
* Models/Mosquito-FB6.xml, mosquito-fbVI-set.xml: change the properties
used for the guns and cannon over multiplayer. Use generic boolean
properties introduced in FlightGear 2017.2.
* Nasal/mosquito.nas: add cannon lights.
* mosquito-common.xml (keyboard): e for guns, E for cannon.