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/component/window.cpp

@@ -0,0 +1,86 @@
+#include <ftxui/component/component.hpp>
+#include <ftxui/component/screen_interactive.hpp>
+
+using namespace ftxui;
+
+Component DummyWindowContent() {
+  class Impl : public ComponentBase {
+   private:
+    bool checked[3] = {false, false, false};
+    float slider = 50;
+
+   public:
+    Impl() {
+      Add(Container::Vertical({
+          Checkbox("Check me", &checked[0]),
+          Checkbox("Check me", &checked[1]),
+          Checkbox("Check me", &checked[2]),
+          Slider("Slider", &slider, 0.f, 100.f),
+      }));
+    }
+  };
+  return Make<Impl>();
+}
+
+int main() {
+
+  int window_1_left = 20;
+  int window_1_top = 10;
+  int window_1_width = 40;
+  int window_1_height = 20;
+
+  auto window_1 = Window({
+      .inner = DummyWindowContent(),
+      .title = "First window",
+      .left = &window_1_left,
+      .top = &window_1_top,
+      .width = &window_1_width,
+      .height = &window_1_height,
+  });
+
+  auto window_2 = Window({
+      .inner = DummyWindowContent(),
+      .title = "My window",
+      .left = 40,
+      .top = 20,
+  });
+
+  auto window_3 = Window({
+      .inner = DummyWindowContent(),
+      .title = "My window",
+      .left = 60,
+      .top = 30,
+  });
+
+  auto window_4 = Window({
+      .inner = DummyWindowContent(),
+  });
+
+  auto window_5 = Window({});
+
+  auto window_container = Container::Stacked({
+      window_1,
+      window_2,
+      window_3,
+      window_4,
+      window_5,
+  });
+
+  auto display_win_1 = Renderer([&] {
+    return text("window_1: " +  //
+                std::to_string(window_1_width) + "x" +
+                std::to_string(window_1_height) + " + " +
+                std::to_string(window_1_left) + "," +
+                std::to_string(window_1_top));
+  });
+
+  auto layout = Container::Vertical({
+      display_win_1,
+      window_container,
+  });
+
+  auto screen = ScreenInteractive::Fullscreen();
+  screen.Loop(layout);
+
+  return EXIT_SUCCESS;
+}