Fix focus vs flexbox interaction. (#405)

- Fix focus in flexbox. This required resetting the focus state at the
  beginning of the ComputeRequirement(), because it can now run several
  times.

  This resolves:https://github.com/ArthurSonzogni/FTXUI/issues/399

- Add Box::Union.

- Add a preliminary implementation of forwarding selected_box from
  within the flexbox.

include/ftxui/screen/box.hpp

@@ -10,6 +10,7 @@ struct Box {
   int y_max = 0;
 
   static auto Intersection(Box a, Box b) -> Box;
+  static auto Union(Box a, Box b) -> Box;
   bool Contain(int x, int y) const;
   bool operator==(const Box& other) const;
   bool operator!=(const Box& other) const;