FlightGear - Flight Simulator FGAddon

This is a major contribution by Alexandre Brenta.

* Models/b25.ac (Dorsal_Turret): new object created from parts of vitreJ.
* Models/Interior/Guns/SideGuns.ac: delete the half guns of the dorsal turret.
* Models/Interior/interior.ac (IntSideGunsBase): rename to Dorsal_Turret_Interior;
move parts of intvitreJ and others into this object.
* Models/Interior/interior.xml: rotate Dorsal_Turret_Interior.
* Models/b25.xml: rotate Dorsal_Turret. Add two M2 Browning maching guns. Rotate
them too, horizontally and vertically.
* Nasal/turrets.nas: new, copy and modify from the B-17. Simplify. Bug fix: use
the view name instead of a nondeterministic view number.
* b25j-set.xml: load it. Add a dorsal turret view.