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/flexbox_test.cpp

@@ -432,6 +432,29 @@ TEST(FlexboxTest, GapY) {
             "       ");
 }
 
+TEST(FlexboxTest, Focus) {
+  auto document = vbox({
+    paragraph("0 -"),
+    paragraph("1 -"),
+    paragraph("2 -"),
+    paragraph("3 -"),
+    paragraph("4 -"),
+    paragraph("5 -"),
+    paragraph("6 -"),
+    paragraph("7 -") | focus,
+    paragraph("8 -"),
+    paragraph("9 -"),
+  }) | yframe | flex;
+
+  Screen screen(1, 3);
+  Render(screen, document);
+  EXPECT_EQ(screen.ToString(),
+            "7\r\n"
+            "-\r\n"
+            "8"
+            );
+}
+
 }  // namespace ftxui
 
 // Copyright 2021 Arthur Sonzogni. All rights reserved.