Reset: event manager can be unbound.

simgear/structure/event_mgr.cxx

@@ -38,6 +38,12 @@ void SGTimer::run()
     (*callback)();
 }
 
+void SGEventMgr::unbind()
+{
+    _freezeProp.clear();
+    _rtProp.clear();
+}
+
 void SGEventMgr::update(double delta_time_sec)
 {
     _simQueue.update(delta_time_sec);

simgear/structure/event_mgr.hxx

@@ -73,7 +73,8 @@ public:
 
     virtual void init() {}
     virtual void update(double delta_time_sec);
-
+    virtual void unbind();
+    
     void setRealtimeProperty(SGPropertyNode* node) { _rtProp = node; }
 
     /**