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();

include/ftxui/component/component_options.hpp

@@ -25,6 +25,7 @@ struct EntryState {
   bool state;         ///< The state of the button/checkbox/radiobox
   bool active;        ///< Whether the entry is the active one.
   bool focused;       ///< Whether the entry is one focused by the user.
+  int index;          ///< Index of the entry when applicable or -1.
 };
 
 struct UnderlineOption {