Add support for nxxm.

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

src/ftxui/dom/dbox.cpp

@@ -0,0 +1,40 @@
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+class DBox : public Node {
+ public:
+  DBox(Elements children) : Node(std::move(children)) {}
+  ~DBox() {}
+
+  void ComputeRequirement() override {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    requirement_.flex.x = 1;
+    requirement_.flex.y = 0;
+    for (auto& child : children) {
+      child->ComputeRequirement();
+      requirement_.min.x = std::max(requirement_.min.x, child->requirement().min.x);
+      requirement_.min.y = std::max(requirement_.min.y, child->requirement().min.y);
+
+      if (requirement_.selection < child->requirement().selection) {
+        requirement_.selection = child->requirement().selection;
+        requirement_.selected_box = child->requirement().selected_box;
+      }
+    }
+  }
+
+  void SetBox(Box box) override {
+    Node::SetBox(box);
+
+    for (auto& child : children)
+      child->SetBox(box);
+  }
+};
+
+std::unique_ptr<Node> dbox(Elements children) {
+  return std::make_unique<DBox>(std::move(children));
+}
+
+};  // namespace ftxui