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.

CHANGELOG.md

@@ -4,6 +4,13 @@ Changelog
 current (development) 
 ---------------------
 
+### DOM
+- Bugfix: Fix `focus`/`select` when the `vbox`/`hbox`/`dbox` contains a
+  `flexbox`
+
+### Screen
+- Feature: add `Box::Union(a,b) -> Box`
+
 3.0.0
 -----