refs #649, with #659 deferred caches available

* adjustments to use deferred caches
* removed simulator from CAircraftModelSetLoader`s signature as it was not used
* only change current simulator when explicitly set (avoid unintended setting)
* added function to obtain number of elements from model caches

src/blackmisc/simulation/aircraftmodelsetloader.h

@@ -46,7 +46,7 @@ namespace BlackMisc
 
         public:
             //! Constructor
-            CAircraftModelSetLoader(const CSimulatorInfo &simulator, QObject *parent = nullptr);
+            CAircraftModelSetLoader(QObject *parent = nullptr);
 
             //! Destructor
             virtual ~CAircraftModelSetLoader();