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.

examples/component/focus.cpp

@@ -32,8 +32,8 @@ Element make_grid() {
 };
 
 int main(int argc, const char* argv[]) {
-  float focus_x = 0.0f;
-  float focus_y = 0.0f;
+  float focus_x = 0.5f;
+  float focus_y = 0.5f;
 
   auto slider_x = Slider("x", &focus_x, 0.f, 1.f, 0.01f);
   auto slider_y = Slider("y", &focus_y, 0.f, 1.f, 0.01f);

examples/component/with_restored_io.cpp

@@ -18,14 +18,12 @@ int main() {
   // temporarily uninstall the terminal hook and execute the provided callback
   // function. This allow running the application in a non-interactive mode.
   auto btn_run = Button("Execute with restored IO", screen.WithRestoredIO([] {
-    std::system("bash");
     std::cout << "This is a child program using stdin/stdout." << std::endl;
     for (int i = 0; i < 10; ++i) {
       std::cout << "Please enter 10 strings (" << i << "/10)" << std::flush;
       std::string input;
       std::getline(std::cin, input);
     }
-    std::system("bash");
   }));
 
   auto btn_quit = Button("Quit", screen.ExitLoopClosure());

examples/dom/color_info_sorted_2d.ipp

@@ -34,7 +34,7 @@ std::vector<std::vector<ColorInfo>> ColorInfoSorted2D() {
     for (int i = 0; i < int(column.size()) - 1; ++i) {
       int best_index = i + 1;
       int best_distance = 255 * 255 * 3;
-      for (size_t j = i + 1; j < column.size(); ++j) {
+      for (int j = i + 1; j < column.size(); ++j) {
         int dx = column[i].red - column[j].red;
         int dy = column[i].green - column[j].green;
         int dz = column[i].blue - column[j].blue;