Add CheckBox.

examples/component/CMakeLists.txt

examples/component/checkbox.cpp

examples/component/gallery.cpp

examples/component/tab_horizontal.cpp

examples/component/tab_vertical.cpp

examples/component/toggle.cpp

ftxui/CMakeLists.txt

ftxui/include/ftxui/component/checkbox.hpp

ftxui/include/ftxui/component/component.hpp

ftxui/include/ftxui/component/container.hpp

ftxui/include/ftxui/component/delegate.hpp

ftxui/include/ftxui/component/menu.hpp

ftxui/include/ftxui/component/toggle.hpp

ftxui/src/ftxui/component/checkbox.cpp

ftxui/src/ftxui/component/component.cpp

ftxui/src/ftxui/component/container.cpp

ftxui/src/ftxui/component/menu.cpp

ftxui/src/ftxui/component/screen_interactive.cpp

ftxui/src/ftxui/component/toggle.cpp

ftxui/src/ftxui/screen/screen.cpp