added save functions to various classes

Each class is responsible for its own saving and loading from the XML data
in order to reduce the complexity of the save code.

app/project/item/folder/folder.cpp

@@ -40,3 +40,16 @@ QIcon Folder::icon()
 {
   return icon::Folder;
 }
+
+void Folder::Save(QXmlStreamWriter *writer) const
+{
+  writer->writeStartElement("folder");
+
+  writer->writeAttribute("name", name());
+
+  foreach (ItemPtr child, children()) {
+    child->Save(writer);
+  }
+
+  writer->writeEndElement(); // folder
+}