Bring back C++17 minimal requirement. (#475)

cmake/ftxui_set_options.cmake

cmake/ftxui_test.cmake

codecov.yml

examples/component/slider_direction.cpp

include/ftxui/screen/screen.hpp

src/ftxui/component/animation.cpp

src/ftxui/component/radiobox.cpp

src/ftxui/component/screen_interactive.cpp

src/ftxui/component/slider.cpp

src/ftxui/screen/color.cpp