refs #395, improvements for snapshot / restricted aircraft handling

* fixed isCurrentThreadCreatingThread and renamed to isCurrentThreadObjectThread (this check was never working), added 2 more thread checks
* changed remove aircraft function to return number of returned aircraft (like with the containers this allows to avoid unnecessary signals)
* removed unused function ps_recalculateRenderedAircraft() / SimulatorCommon
* using Queued airspaceAircraftSnapshot signal for binding (functor connect does not provide connection type)
* extened ASSERTs to check threads
* simulator: initial situation function with return value (success?)
* simulator: avoid unneccessary copy and provide correct rendered flag in add aircraft function

src/plugins/simulator/xplane/simulator_xplane.cpp

@@ -393,11 +393,14 @@ namespace BlackSimPlugin
             return true;
         }
 
-        void CSimulatorXPlane::physicallyRemoveAllRemoteAircraft()
+        int CSimulatorXPlane::physicallyRemoveAllRemoteAircraft()
         {
+            //! \todo XP driver obtain number of removed aircraft
+            int r = getAircraftInRangeCount();
             m_traffic->removeAllPlanes();
             updateMarkAllAsNotRendered(simulatorOriginator());
             CLogMessage(this).info("XP: Removed all aircraft");
+            return r;
         }
 
         CCallsignSet CSimulatorXPlane::physicallyRenderedAircraft() const