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.

src/ftxui/component/screen_interactive.cpp

@@ -297,8 +297,9 @@ ScreenInteractive ScreenInteractive::FitComponent() {
 void ScreenInteractive::Post(Task task) {
   // Task/Events sent toward inactive screen or screen waiting to become
   // inactive are dropped.
-  if (!task_sender_)
+  if (!task_sender_) {
     return;
+  }
 
   task_sender_->Send(std::move(task));
 }