add_missing_frames: optimize.
* run after part insertion, within json_load/load_legacy functions
* push new frame into item list instead of using vehicle::install_part, which triggers
  find_external_parts, find_exhaust, precalc_mounts once for every frame added; load
  does this anyway.
* Only run if savegame_loading_version is v10 or below; new saves won't have vehicles
  with invalid frame config.