Add support for nxxm.

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

include/ftxui/screen/screen.hpp

@@ -0,0 +1,72 @@
+#ifndef FTXUI_SCREEN_SCREEN
+#define FTXUI_SCREEN_SCREEN
+
+#include <string>
+#include <vector>
+#include <memory>
+
+#include "ftxui/screen/color.hpp"
+#include "ftxui/screen/box.hpp"
+
+namespace ftxui {
+  class Node;
+}
+
+namespace ftxui {
+
+struct Pixel {
+  wchar_t character = U' ';
+  bool blink = false;
+  bool bold = false;
+  bool dim = false;
+  bool inverted = false;
+  bool underlined = false;
+  Color background_color = Color::Default;
+  Color foreground_color = Color::Default;
+};
+
+struct Dimension {
+  static Dimension Fixed(int);
+  static Dimension Fit(std::unique_ptr<Node>&);
+  static Dimension Full();
+
+  int dimx;
+  int dimy;
+};
+
+class Screen {
+ public:
+  // Constructor.
+  Screen(int dimx, int dimy);
+  static Screen Create(Dimension dimension);
+  static Screen Create(Dimension width, Dimension height);
+
+  // Node write into the screen using Screen::at.
+  wchar_t& at(int x, int y);
+  Pixel& PixelAt(int x, int y);
+
+  // Convert the screen into a printable string in the terminal.
+  std::string ToString();
+
+  // Get screen dimensions.
+  int dimx() { return dimx_;}
+  int dimy() { return dimy_;}
+
+  // Move the terminal cursor n-lines up with n = dimy().
+  std::string ResetPosition();
+
+  // Fill with space.
+  void Clear();
+
+  void ApplyShader();
+  Box stencil;
+
+ protected:
+  int dimx_;
+  int dimy_;
+  std::vector<std::vector<Pixel>> pixels_;
+};
+
+};  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_SCREEN_SCREEN */