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/resizable_split.cpp

@@ -94,7 +94,7 @@ class ResizableSplitBase : public ComponentBase {
     return false;
   }
 
-  Element Render() final {
+  Element OnRender() final {
     switch (options_->direction()) {
       case Direction::Left:
         return RenderLeft();