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/elements.hpp

@@ -161,7 +161,7 @@ Element frame(Element);
 Element xframe(Element);
 Element yframe(Element);
 Element focus(Element);
-Element select(Element);
+Element select(Element e);  // Deprecated - Alias for focus.
 
 // --- Cursor ---
 // Those are similar to `focus`, but also change the shape of the cursor.

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_;

include/ftxui/dom/requirement.hpp

@@ -5,8 +5,10 @@
 #define FTXUI_DOM_REQUIREMENT_HPP
 
 #include "ftxui/screen/box.hpp"
+#include "ftxui/screen/screen.hpp"
 
 namespace ftxui {
+class Node;
 
 struct Requirement {
   // The required size to fully draw the element.
@@ -20,13 +22,28 @@ struct Requirement {
   int flex_shrink_y = 0;
 
   // Focus management to support the frame/focus/select element.
-  enum Selection {
-    NORMAL = 0,
-    SELECTED = 1,
-    FOCUSED = 2,
+  struct Focused {
+    bool enabled = false;
+    Box box;
+    Node* node = nullptr;
+    Screen::Cursor::Shape cursor_shape = Screen::Cursor::Shape::Hidden;
+
+    // Internal for interactions with components.
+    bool component_active = false;
+
+    // Return whether this requirement should be preferred over the other.
+    bool Prefer(const Focused& other) const {
+      if (!other.enabled) {
+        return false;
+      }
+      if (!enabled) {
+        return true;
+      }
+
+      return other.component_active && !component_active;
+    }
   };
-  Selection selection = NORMAL;
-  Box selected_box;
+  Focused focused;
 };
 
 }  // namespace ftxui