Support SIGTSTP and task posting. (#331)

- Add support for SIGTSTP:
https://github.com/ArthurSonzogni/FTXUI/issues/330
This

- Add support for task posting.
This allows folks to defer function execution, and execute it directly
below the main loop. The task are executed in a FIFO order.

include/ftxui/component/event.hpp

@@ -2,7 +2,8 @@
 #define FTXUI_COMPONENT_EVENT_HPP
 
 #include <ftxui/component/mouse.hpp>  // for Mouse
-#include <string>                     // for string, operator==
+#include <functional>
+#include <string>  // for string, operator==
 #include <vector>
 
 namespace ftxui {