Merge dom and component focus (#978)

Instead of two levels of focus with `focus` and `selected`, use a recursive
level. The components set the one "active" and hbox/vbox/dbox 

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/component/window.cpp

@@ -124,7 +124,7 @@ class WindowImpl : public ComponentBase, public WindowOptions {
   }
 
  private:
-  Element Render() final {
+  Element OnRender() final {
     auto element = ComponentBase::Render();
 
     const bool captureable =