Add Event.

examples/CMakeLists.txt

examples/input/CMakeLists.txt

examples/input/main.cpp

examples/print_key_press/main.cpp

examples/toggle/main.cpp

ftxui/CMakeLists.txt

ftxui/include/ftxui/component/component.hpp

ftxui/include/ftxui/component/component_direction.hpp

ftxui/include/ftxui/component/component_horizontal.hpp

ftxui/include/ftxui/component/component_vertical.hpp

ftxui/include/ftxui/component/input.hpp

ftxui/include/ftxui/component/menu.hpp

ftxui/include/ftxui/component/toggle.hpp

ftxui/include/ftxui/event.hpp

ftxui/src/ftxui/component/component.cpp

ftxui/src/ftxui/component/component_direction.cpp

ftxui/src/ftxui/component/component_horizontal.cpp

ftxui/src/ftxui/component/component_vertical.cpp

ftxui/src/ftxui/component/input.cpp

ftxui/src/ftxui/component/menu.cpp