Add index to EntryState (#933)

CHANGELOG.md

include/ftxui/component/component_base.hpp

include/ftxui/component/component_options.hpp

src/ftxui/component/button.cpp

src/ftxui/component/checkbox.cpp

src/ftxui/component/component.cpp

src/ftxui/component/menu.cpp

src/ftxui/component/menu_test.cpp

src/ftxui/component/radiobox.cpp