Track energy separately for weapons, engines

[JeremyLeland]

Probably simpler to have separate energy pools for weapons and engines, and maybe even per weapon.

Combine the bullet offsets into a single weapon (if they use the same ammo)? Probably want to track different weapon systems separately anyway. Then we could also show the energy available for given weapon more clearly in UI. ("Energy" is basically "ammo" in this case, so maybe its always reloading?)

Want to avoid situation where we fire half our bullets -- should be all or nothing.