Add support for nxxm.

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

src/ftxui/dom/border.cpp

@@ -0,0 +1,118 @@
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+using namespace ftxui;
+
+static wchar_t simple_border_charset[] = L"┌┐└┘─│┬┴┤├";
+
+class Border : public Node {
+ public:
+  Border(Elements children)
+      : Node(std::move(children)),
+        charset(std::begin(simple_border_charset),
+                std::end(simple_border_charset)) {}
+  Border(Elements children, Pixel pixel)
+      : Node(std::move(children)), charset_pixel(10, pixel) {}
+  ~Border() override {}
+
+  std::vector<Pixel> charset_pixel; 
+  std::vector<wchar_t> charset;
+
+  void ComputeRequirement() override {
+    Node::ComputeRequirement();
+    requirement_ = children[0]->requirement();
+    requirement_.min.x += 2;
+    requirement_.min.y += 2;
+    if (children.size() == 2) {
+      requirement_.min.x =
+          std::max(requirement_.min.x, children[1]->requirement().min.x + 2);
+    }
+    requirement_.selected_box.x_min++;
+    requirement_.selected_box.x_max++;
+    requirement_.selected_box.y_min++;
+    requirement_.selected_box.y_max++;
+  }
+
+  void SetBox(Box box) override {
+    Node::SetBox(box);
+    if (children.size() == 2) {
+      Box title_box;
+      title_box.x_min = box.x_min + 1;
+      title_box.x_max = box.x_max - 1;
+      title_box.y_min = box.y_min;
+      title_box.y_max = box.y_min;
+      children[1]->SetBox(title_box);
+    }
+    box.x_min++;
+    box.x_max--;
+    box.y_min++;
+    box.y_max--;
+    children[0]->SetBox(box);
+  }
+
+  void Render(Screen& screen) override {
+    // Draw content.
+    children[0]->Render(screen);
+
+    // Draw the border.
+    if (box_.x_min >= box_.x_max || box_.y_min >= box_.y_max)
+      return;
+
+    if (!charset.empty())
+      RenderPixel(screen);
+    else
+      RenderChar(screen);
+  }
+
+  void RenderPixel(Screen& screen) {
+    screen.at(box_.x_min, box_.y_min) = charset[0];
+    screen.at(box_.x_max, box_.y_min) = charset[1];
+    screen.at(box_.x_min, box_.y_max) = charset[2];
+    screen.at(box_.x_max, box_.y_max) = charset[3];
+    for(float x = box_.x_min + 1; x<box_.x_max; ++x) {
+      screen.at(x, box_.y_min) = charset[4];
+      screen.at(x, box_.y_max) = charset[4];
+    }
+    for(float y = box_.y_min + 1; y<box_.y_max; ++y) {
+      screen.at(box_.x_min, y) = charset[5];
+      screen.at(box_.x_max,y) = charset[5];
+    }
+
+    // Draw title.
+    if (children.size() == 2)
+      children[1]->Render(screen);
+  }
+
+  void RenderChar(Screen& screen) {
+    screen.PixelAt(box_.x_min, box_.y_min) = charset_pixel[0];
+    screen.PixelAt(box_.x_max, box_.y_min) = charset_pixel[1];
+    screen.PixelAt(box_.x_min, box_.y_max) = charset_pixel[2];
+    screen.PixelAt(box_.x_max, box_.y_max) = charset_pixel[3];
+    for(float x = box_.x_min + 1; x<box_.x_max; ++x) {
+      screen.PixelAt(x, box_.y_min) = charset_pixel[4];
+      screen.PixelAt(x, box_.y_max) = charset_pixel[4];
+    }
+    for(float y = box_.y_min + 1; y<box_.y_max; ++y) {
+      screen.PixelAt(box_.x_min, y) = charset_pixel[5];
+      screen.PixelAt(box_.x_max,y) = charset_pixel[5];
+    }
+  }
+};
+
+std::unique_ptr<Node> border(Element child) {
+  return std::make_unique<Border>(unpack(std::move(child)));
+}
+
+std::unique_ptr<Node> window(Element title, Element content) {
+  return std::make_unique<Border>(unpack(std::move(content), std::move(title)));
+}
+
+Decorator borderWith(Pixel pixel) {
+  return [pixel](Element child) {
+    return std::make_unique<Border>(unpack(std::move(child)), pixel);
+  };
+}
+
+};  // namespace ftxui