Save id in attributes in state saving.

Element.cpp

@@ -59,6 +59,21 @@ void Element::setOpacity(float opacity)
   }
 }
 
+void Element::read(const QDomElement& obj)
+{
+  Serializable::read(obj);
+
+  // Check id.
+  Q_ASSERT(_id == obj.attribute("id").toInt());
+}
+
+void Element::write(QDomElement& obj)
+{
+  Serializable::write(obj);
+  // Set id.
+  obj.setAttribute("id", getId());
+}
+
 void Element::_emitPropertyChanged(const QString& propertyName)
 {
   emit propertyChanged(getId(), propertyName, property(propertyName.toAscii()));

Element.h

@@ -63,6 +63,9 @@ public:
 
   virtual QIcon getIcon() const { return QIcon(); }
 
+  virtual void read(const QDomElement& obj);
+  virtual void write(QDomElement& obj);
+
 signals:
   void propertyChanged(uid id, QString propertyName, QVariant value);