Add support for nxxm.

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

include/ftxui/dom/elements.hpp

@@ -0,0 +1,88 @@
+#ifndef FTXUI_DOM_ELEMENTS_HPP
+#define FTXUI_DOM_ELEMENTS_HPP
+
+#include <functional>
+
+#include "ftxui/dom/node.hpp"
+#include "ftxui/screen/color.hpp"
+
+namespace ftxui {
+
+using Element = std::unique_ptr<Node>;
+using Elements = std::vector<Element>;
+using Decorator = std::function<Element(Element)>;
+using GraphFunction = std::function<std::vector<int>(int,int)>;
+
+// --- Widget ---
+Element text(std::wstring text);
+Element separator();
+Element separator(Pixel);
+Element gauge(float ratio);
+Element border(Element);
+Decorator borderWith(Pixel);
+Element window(Element title, Element content);
+Element spinner(int charset_index, size_t image_index);
+Elements paragraph(std::wstring text); // Use inside hflow(). Split by space.
+Element graph(GraphFunction);
+
+// -- Decorator ---
+Element bold(Element);
+Element dim(Element);
+Element inverted(Element);
+Element underlined(Element);
+Element blink(Element);
+Decorator color(Color);
+Decorator bgcolor(Color);
+Element color(Color, Element);
+Element bgcolor(Color, Element);
+
+// --- Layout ---
+// Horizontal, Vertical or stacked set of elements.
+Element hbox(Elements);
+Element vbox(Elements);
+Element dbox(Elements);
+Element hflow(Elements);
+
+// -- Flexibility ---
+// Define how to share the remaining space when not all of it is used inside a
+// container.
+Element filler();
+Element flex(Element);
+Element notflex(Element);
+
+// -- Size override;
+enum Direction { WIDTH, HEIGHT };
+enum Constraint { LESS_THAN, EQUAL, GREATER_THAN };
+Decorator size(Direction, Constraint, int value);
+
+// --- Frame ---
+// A frame is a scrollable area. The internal area is potentially larger than
+// the external one. The internal area is scrolled in order to make visible the
+// focused element.
+Element frame(Element);
+Element focus(Element);
+Element select(Element);
+
+// --- Util --------------------------------------------------------------------
+Element hcenter(Element);
+Element vcenter(Element);
+Element center(Element);
+Element align_right(Element);
+Element nothing(Element element);
+
+// Pipe elements into decorator togethers.
+// Examples: text("ftxui") | bold | underlined;
+Element operator|(Element, Decorator);
+Elements operator|(Elements, Decorator);
+Decorator operator|(Decorator, Decorator);
+
+// Make container able to take any number of children as input.
+#include "take_any_args.hpp"
+TAKE_ANY_ARGS(vbox)
+TAKE_ANY_ARGS(hbox)
+TAKE_ANY_ARGS(dbox)
+TAKE_ANY_ARGS(hflow)
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_DOM_ELEMENTS_HPP */