Add index to EntryState (#933)

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

include/ftxui/component/component_base.hpp

@@ -44,6 +44,7 @@ class ComponentBase {
   ComponentBase* Parent() const;
   Component& ChildAt(size_t i);
   size_t ChildCount() const;
+  int Index() const;
   void Add(Component children);
   void Detach();
   void DetachAllChildren();