Add TakeFocus and SetActiveChild.

This allows developers to set child children component must be the
currently active/focused one.

This can be used to "control" where the focus is, without user
interactions.

include/ftxui/component/component.hpp

@@ -39,11 +39,18 @@ class Component {
   // We say an element has the focus if the chain of ActiveChild() from the
   // root component contains this object.
   virtual Component* ActiveChild();
+
   // Whether this is the active child of its parent.
   bool Active();
   // Whether all the ancestors are active.
   bool Focused();
 
+  // Make the |child| to be the "active" one.
+  virtual void SetActiveChild(Component* child);
+
+  // Configure all the ancestors to give focus to this component.
+  void TakeFocus();
+
  private:
   Component* parent_ = nullptr;
   void Detach();

include/ftxui/component/container.hpp

@@ -18,6 +18,7 @@ class Container : public Component {
   bool OnEvent(Event event) override;
   Element Render() override;
   Component* ActiveChild() override;
+  virtual void SetActiveChild(Component*) override;
 
  protected:
   // Handlers