Avoid potential issues with initializing object IDs

When child ObjectGroup layers would be added to a GroupLayer instance
before it is added to the map, the objects in the ObjectGroup would not
get their IDs initialized.

Delegate the setting of map ownership to the Map class, so that the IDs
will initialize also in such a scenario.

src/libtiled/grouplayer.cpp

@@ -49,11 +49,11 @@ void GroupLayer::insertLayer(int index, Layer *layer)
 void GroupLayer::adoptLayer(Layer *layer)
 {
     layer->setParentLayer(this);
-    layer->setMap(map());
 
     if (map())
-        if (ObjectGroup *group = layer->asObjectGroup())
-            map()->initializeObjectIds(*group);
+        map()->adoptLayer(layer);
+    else
+        layer->setMap(nullptr);
 }
 
 Layer *GroupLayer::takeLayerAt(int index)
@@ -115,10 +115,15 @@ Layer *GroupLayer::clone() const
 
 void GroupLayer::setMap(Map *map)
 {
-    // todo: What about initializing object IDs?
     Layer::setMap(map);
-    for (Layer *layer : mLayers)
-        layer->setMap(map);
+
+    if (map) {
+        for (Layer *layer : mLayers)
+            map->adoptLayer(layer);
+    } else {
+        for (Layer *layer : mLayers)
+            layer->setMap(nullptr);
+    }
 }
 
 GroupLayer *GroupLayer::initializeClone(GroupLayer *clone) const

src/libtiled/map.h

@@ -369,6 +369,8 @@ class TILEDSHARED_EXPORT Map : public Object
     void initializeObjectIds(ObjectGroup &objectGroup);
 
 private:
+    friend class GroupLayer;    // so it cal call adoptLayer
+
     void adoptLayer(Layer *layer);
 
     void recomputeDrawMargins() const;