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/screen/terminal.cpp

@@ -20,8 +20,8 @@ namespace ftxui {
 
 namespace {
 
-bool g_cached = false;
-Terminal::Color g_cached_supported_color;
+bool g_cached = false;                     // NOLINT
+Terminal::Color g_cached_supported_color;  // NOLINT
 
 Dimensions& FallbackSize() {
 #if defined(__EMSCRIPTEN__)