Track NPC clones

Manual state management in python was kind of fiddly, so let's track all
avatar clones in the NetApp and unload them as needed. This also seems to
fix a potential bug in plNPCSpawnMod (is that even used?).

Sources/Plasma/PubUtilLib/plNetClient/plNetCliAgeLeaver.cpp

@@ -223,6 +223,7 @@ void plNCAgeLeaver::ExecNextOp () {
             plAgeLoader::GetInstance()->UnloadAge();            // unload age
             nc->ISendCameraReset(false/*leaving age*/);         // reset camera
             nc->IUnloadRemotePlayers();                         // unload other players
+            nc->IUnloadNPCs();                                  // unload non-player clones
 
             if (NetCommNeedToLoadAvatar())
                 am->UnLoadLocalPlayer();