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/task.hpp

@@ -0,0 +1,12 @@
+#include <functional>
+#include <variant>
+#include "ftxui/component/event.hpp"
+
+namespace ftxui {
+using Closure = std::function<void()>;
+using Task = std::variant<Event, Closure>;
+}  // namespace ftxui
+
+// Copyright 2022 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.