Add support for XML content in nodes

Only the XML content that appears as first child of a node is
considered.
If the XML content appears after the first child (no counting
comments), it is ignored.

The XML content is added to the NodeConfig structure under the
`xml_content` field.

include/behaviortree_cpp/tree_node.h

@@ -84,6 +84,8 @@ struct NodeConfig
   PortsRemapping input_ports;
   // output ports
   PortsRemapping output_ports;
+  // Node content (in the XML sense).
+  std::string xml_content;
 
   const TreeNodeManifest* manifest = nullptr;
 

src/xml_parsing.cpp

@@ -697,6 +697,13 @@ TreeNode::Ptr XMLParser::PImpl::createNodeFromXML(const XMLElement* element,
       }
     }
 
+    // Get the XML content of the node.
+    const char* xml_content = element->GetText();
+    if (xml_content != nullptr)
+    {
+      config.xml_content = xml_content;
+    }
+
     new_node = factory.instantiateTreeNode(instance_name, type_ID, config);
   }