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/screen_interactive.hpp

@@ -7,9 +7,11 @@
 #include <memory>                        // for shared_ptr
 #include <string>                        // for string
 #include <thread>                        // for thread
+#include <variant>                       // for variant
 
 #include "ftxui/component/captured_mouse.hpp"  // for CapturedMouse
 #include "ftxui/component/event.hpp"           // for Event
+#include "ftxui/component/task.hpp"            // for Closure, Task
 #include "ftxui/screen/screen.hpp"             // for Screen
 
 namespace ftxui {
@@ -17,37 +19,38 @@ class ComponentBase;
 struct Event;
 
 using Component = std::shared_ptr<ComponentBase>;
+class ScreenInteractivePrivate;
 
 class ScreenInteractive : public Screen {
  public:
-  using Callback = std::function<void()>;
-
   static ScreenInteractive FixedSize(int dimx, int dimy);
   static ScreenInteractive Fullscreen();
   static ScreenInteractive FitComponent();
   static ScreenInteractive TerminalOutput();
 
   void Loop(Component);
-  Callback ExitLoopClosure();
+  Closure ExitLoopClosure();
 
+  void Post(Task task);
   void PostEvent(Event event);
+
   CapturedMouse CaptureMouse();
 
   // Decorate a function. The outputted one will execute similarly to the
   // inputted one, but with the currently active screen terminal hooks
   // temporarily uninstalled.
-  Callback WithRestoredIO(Callback);
+  Closure WithRestoredIO(Closure);
 
  private:
   void Install();
   void Uninstall();
 
   void Main(Component component);
-  ScreenInteractive* suspended_screen_ = nullptr;
 
   void Draw(Component component);
-  void EventLoop(Component component);
+  void SigStop();
 
+  ScreenInteractive* suspended_screen_ = nullptr;
   enum class Dimension {
     FitComponent,
     Fixed,
@@ -61,8 +64,8 @@ class ScreenInteractive : public Screen {
                     Dimension dimension,
                     bool use_alternative_screen);
 
-  Sender<Event> event_sender_;
-  Receiver<Event> event_receiver_;
+  Sender<Task> task_sender_;
+  Receiver<Task> task_receiver_;
 
   std::string set_cursor_position;
   std::string reset_cursor_position;
@@ -75,6 +78,13 @@ class ScreenInteractive : public Screen {
 
   bool mouse_captured = false;
   bool previous_frame_resized_ = false;
+
+ public:
+  class Private {
+   public:
+    static void SigStop(ScreenInteractive& s) { return s.SigStop(); }
+  };
+  friend Private;
 };
 
 }  // namespace ftxui