Merge the old SUBSYSTEM_SHIELD_GENERATOR code

In the old fs2_open-unstable branch, I have an implementation of shield generators, which was originally removed because you can't add a new subsystem type without breaking pilot files. With the new 3.7.0 pilot file code, this restriction no longer applies.

There are a few other goodies in fs2_open-unstable which ought to be investigated and merged as well. I think WMC committed a major update of the camera code.