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/component/component_base.hpp

@@ -50,7 +50,10 @@ class ComponentBase {
   void DetachAllChildren();
 
   // Renders the component.
-  virtual Element Render();
+  Element Render();
+
+  // Override this function modify how `Render` works.
+  virtual Element OnRender();
 
   // Handles an event.
   // By default, reduce on children with a lazy OR.
@@ -94,6 +97,7 @@ class ComponentBase {
 
  private:
   ComponentBase* parent_ = nullptr;
+  bool in_render = false;
 };
 
 }  // namespace ftxui