Add support for nxxm.

[nxxm](https://nxxm.github.io)

src/ftxui/dom/node_decorator.hpp

@@ -0,0 +1,20 @@
+#ifndef FTXUI_DOM_NODE_DECORATOR_H_
+#define FTXUI_DOM_NODE_DECORATOR_H_
+
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+// Helper class.
+class NodeDecorator : public Node {
+ public:
+  NodeDecorator(Elements children) : Node(std::move(children)) {}
+  ~NodeDecorator() override {}
+  void ComputeRequirement() override;
+  void SetBox(Box box) override;
+};
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_DOM_NODE_DECORATOR_H_ */