change XNode children to a vector.

This means the order of children is preserved.

It also means the name of each node isn't duplicated.  This was
an opportunity for an inconsistent XNode tree.  Nodes can now be
renamed after being inserted into a tree.

Searching for children by name isn't efficient.  Use attributes for that.

stepmania/src/ActorUtil.cpp

@@ -232,8 +232,7 @@ namespace
 		{
 			pNode = queue.back();
 			queue.pop_back();
-			FOREACH_Child( pNode, pChild )
-				queue.push_back( pChild );
+			queue.insert( queue.end(), pNode->m_childs.begin(), pNode->m_childs.end() );
 
 			/* Source file, for error messages: */
 			pNode->AppendAttr( "_Source", sFile );