Menu of components (#131)

Allow Container::Vertical and Container::Horizontal to have an
external selector, similar to Container::Tab.

This is useful for implementing a menu of menu.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/dom/separator.cpp

@@ -1,4 +1,5 @@
 #include <memory>  // for make_shared
+#include <string>  // for wstring
 
 #include "ftxui/dom/elements.hpp"     // for Element, separator
 #include "ftxui/dom/node.hpp"         // for Node