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/dom/dbox.cpp

@@ -21,6 +21,7 @@ class DBox : public Node {
     requirement_.flex_grow_y = 0;
     requirement_.flex_shrink_x = 0;
     requirement_.flex_shrink_y = 0;
+    requirement_.selection = Requirement::NORMAL;
     for (auto& child : children_) {
       child->ComputeRequirement();
       requirement_.min_x =