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/CMakeLists.txt

@@ -43,4 +43,5 @@ example(tab_horizontal)
 example(tab_vertical)
 example(textarea)
 example(toggle)
+example(window)
 example(with_restored_io)