server: TrainVehicleList more strict checks for add/remove

- Do not add vehicle to an active train if it is already
  in an other active train

- When removing vehicle from active train reset
  activeTrain property

- When adding vehicle to active train, set it's activeTrain
  property

server/src/train/trainvehiclelist.cpp

@@ -170,8 +170,8 @@ void TrainVehicleList::rowsChanged(uint32_t first, uint32_t last)
 
 void TrainVehicleList::addObject(std::shared_ptr<RailVehicle> vehicle)
 {
-  if(!vehicle)
-    return; //Cannot add null vehicles
+  if(!vehicle || (vehicle->activeTrain.value() && train().active))
+    return; //Cannot add null vehicles or vehicles in other active trains to an active train
 
   std::shared_ptr<TrainVehicleListItem> object = getItemFromVehicle(vehicle);
   if(object)
@@ -181,6 +181,9 @@ void TrainVehicleList::addObject(std::shared_ptr<RailVehicle> vehicle)
   object->vehicle.setValueInternal(vehicle);
   object->vehicle->trains.appendInternal(parent().shared_ptr<Train>());
 
+  if(train().active)
+    object->vehicle->activeTrain.setValueInternal(parent().shared_ptr<Train>());
+
   items.appendInternal(object);
   m_propertyChanged.emplace(object.get(), object->propertyChanged.connect(std::bind(&TrainVehicleList::propertyChanged, this, std::placeholders::_1)));
 
@@ -197,6 +200,9 @@ void TrainVehicleList::removeObject(const std::shared_ptr<TrainVehicleListItem>
   m_propertyChanged[item.get()].disconnect();
   m_propertyChanged.erase(item.get());
 
+  if(item->vehicle->activeTrain.value() == parent().shared_ptr<Train>())
+    item->vehicle->activeTrain.setValueInternal(nullptr);
+
   item->vehicle->trains.removeInternal(parent().shared_ptr<Train>());
   item->destroy();
   items.removeInternal(item);