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>

include/ftxui/dom/node.hpp

@@ -59,6 +59,8 @@ class Node {
   };
   virtual void Check(Status* status);
 
+  friend void Render(Screen& screen, Node* node, Selection& selection);
+
  protected:
   Elements children_;
   Requirement requirement_;