Introduce gridbox. (#190)

Introduce gridbox.
Similar to hbox and vbox, this component combine both into a grid.

examples/dom/CMakeLists.txt

@@ -19,6 +19,7 @@ example(color_truecolor_RGB)
 example(color_truecolor_HSV)
 example(color_info_palette256)
 example(style_dim)
+example(gridbox)
 example(style_gallery)
 example(style_inverted)
 example(style_underlined)

examples/dom/gridbox.cpp

@@ -0,0 +1,49 @@
+#include <stdio.h>                  // for getchar
+#include <ftxui/dom/elements.hpp>   // for filler, text, hbox, vbox
+#include <ftxui/screen/screen.hpp>  // for Full, Screen
+#include <memory>                   // for allocator
+
+#include "ftxui/dom/node.hpp"    // for Render
+#include "ftxui/screen/box.hpp"  // for ftxui
+
+int main(int argc, const char* argv[]) {
+  using namespace ftxui;
+  auto cell = [](const char* t) { return text(t) | border; };
+  auto document =  //
+      gridbox({
+          {
+              cell("north-west"),
+              cell("north"),
+              cell("north-east"),
+          },
+          {
+              cell("center-west"),
+              gridbox({
+                  {
+                      cell("center-north-west"),
+                      cell("center-north-east"),
+                  },
+                  {
+                      cell("center-south-west"),
+                      cell("center-south-east"),
+                  },
+              }),
+              cell("center-east"),
+          },
+          {
+              cell("south-west"),
+              cell("south"),
+              cell("south-east"),
+          },
+      });
+  auto screen = Screen::Create(Dimension::Fit(document));
+  Render(screen, document);
+  screen.Print();
+  getchar();
+
+  return 0;
+}
+
+// Copyright 2021 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.