Feature: Windows. (#690)

Into ftxui/component/, add:
```
Container::Stacked(...)
Window(...);
```

Together, they can be used to display draggable/resizable windows.

Bug:https://github.com/ArthurSonzogni/FTXUI/issues/682

* Fix typo.

examples/dom/CMakeLists.txt

@@ -35,4 +35,3 @@ example(style_underlined_double)
 example(table)
 example(vbox_hbox)
 example(vflow)
-example(window)

examples/dom/window.cpp

@@ -1,34 +0,0 @@
-#include <stdlib.h>                // for EXIT_SUCCESS
-#include <ftxui/dom/elements.hpp>  // for operator|=, Element, bgcolor, color, graph, border
-#include <ftxui/screen/screen.hpp>  // for Fixed, Screen
-#include <vector>                   // for vector
-
-#include "ftxui/dom/node.hpp"      // for Render
-#include "ftxui/screen/color.hpp"  // for Color, Color::DarkBlue, Color::Red, ftxui
-
-int main() {
-  using namespace ftxui;
-  Element document = graph([](int x, int y) {
-    std::vector<int> result(x, 0);
-    for (int i{0}; i < x; ++i) {
-      result[i] = ((3 * i) / 2) % y;
-    }
-    return result;
-  });
-
-  document |= color(Color::Red);
-  document |= bgcolor(Color::DarkBlue);
-  document |= border;
-
-  const int width = 80;
-  const int height = 10;
-  auto screen =
-      Screen::Create(Dimension::Fixed(width), Dimension::Fixed(height));
-  Render(screen, document);
-  screen.Print();
-  return EXIT_SUCCESS;
-}
-
-// Copyright 2020 Arthur Sonzogni. All rights reserved.
-// Use of this source code is governed by the MIT license that can be found in
-// the LICENSE file.