BugID: 258

The most save solution for this problem is to perform the real unload action in the Navigator context (thus not in a seperate thread). For this purpose the so far unused message DPE of the GCFNavigatorInstance DPType will be used. The usage of the navPMLunloadPropertySet stays the same but this method "sends" a message to the message DPE of the current naviagtor instance. The callback than performs the real unload actions.