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/modal.cpp

@@ -26,7 +26,7 @@ Component Modal(Component main, Component modal, const bool* show_modal) {
     }
 
    private:
-    Element Render() override {
+    Element OnRender() override {
       selector_ = *show_modal_;
       auto document = main_->Render();
       if (*show_modal_) {