Cessna-208-Caravan: add sim/model/fallback-model-index.
CitationX: add sim/model/fallback-model-index (Fall back on the BAe 125).
787-8: add sim/model/fallback-model-index.
dhc6: add sim/model/fallback-model-index.
Q400: correct sim/model/fallback-model-index.
737-800: correct sim/model/fallback-model-index.
737-300: correct sim/model/fallback-model-index.
cri-cri: correct sim/model/fallback-model-index.
747-8i: correct sim/model/fallback-model-index.
747-400: correct sim/model/fallback-model-index.
