execute IWYU and add some coverage tests.

src/ftxui/component/animation.cpp

src/ftxui/component/animation_test.cpp

src/ftxui/component/input.cpp

src/ftxui/component/menu.cpp

src/ftxui/component/menu_test.cpp

src/ftxui/component/radiobox.cpp

src/ftxui/component/terminal_input_parser_test.cpp

src/ftxui/dom/blink_test.cpp

src/ftxui/dom/bold_test.cpp

src/ftxui/dom/border_test.cpp

src/ftxui/dom/canvas_test.cpp

src/ftxui/dom/color_test.cpp

src/ftxui/dom/dim_test.cpp

src/ftxui/dom/flexbox_helper.cpp

src/ftxui/dom/scroll_indicator_test.cpp

src/ftxui/dom/separator_test.cpp

src/ftxui/dom/spinner_test.cpp

src/ftxui/dom/underlined_test.cpp

src/ftxui/screen/color_test.cpp

src/ftxui/screen/string_test.cpp