Add support for nxxm.

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

include/ftxui/component/component.hpp

@@ -0,0 +1,56 @@
+#ifndef FTXUI_COMPONENT_COMPONENT_HPP
+#define FTXUI_COMPONENT_COMPONENT_HPP
+
+#include "ftxui/component/event.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+class Delegate;
+class Focus;
+
+class Component {
+ public:
+
+  // Constructor/Destructor.
+  Component() = default;
+  virtual ~Component();
+
+  // Component hierarchy.
+  Component* Parent() { return parent_; }
+  void Add(Component* children);
+
+  // Renders the component.
+  virtual Element Render();
+
+  // Handles an event.
+  // By default, reduce on children with a lazy OR.
+  //
+  // Returns whether the event was handled or not.
+  virtual bool OnEvent(Event);
+
+  // Focus management ----------------------------------------------------------
+  //
+  // If this component contains children, this indicates which one is active,
+  // nullptr if none is active.
+  //
+  // We say an element has the focus if the chain of ActiveChild() from the
+  // root component contains this object.
+  virtual Component* ActiveChild();
+  // Whether this is the active child of its parent.
+  bool Active();
+  // Whether all the ancestors are active.
+  bool Focused();
+
+ private:
+  Component* parent_ = nullptr;
+  void Detach();
+  void Attach(Component* parent);
+
+ protected:
+  std::vector<Component*> children_;
+};
+
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_COMPONENT_COMPONENT_HPP */