Add {Const,}StringRef to simplify components.

examples/component/button.cpp

examples/component/checkbox.cpp

examples/component/checkbox_in_frame.cpp

examples/component/gallery.cpp

examples/component/homescreen.cpp

examples/component/input.cpp

examples/component/menu2.cpp

examples/component/menu_style.cpp

examples/component/modal_dialog.cpp

examples/component/radiobox_in_frame.cpp

examples/component/slider.cpp

examples/component/slider_rgb.cpp

examples/component/tab_horizontal.cpp

examples/component/tab_vertical.cpp

examples/component/toggle.cpp

include/ftxui/component/button.hpp

include/ftxui/component/checkbox.hpp

include/ftxui/component/component.hpp

include/ftxui/component/input.hpp

include/ftxui/screen/string.hpp