Implement and document Fl::add_system_handler() for Wayland

src/Fl.cxx

@@ -960,7 +960,7 @@ static system_handler_link *sys_handlers = 0;
    - X11: XEvent
    - Windows: MSG
    - OS X: NSEvent
-   - Wayland: not implemented
+   - Wayland: NULL (FLTK runs the event handler(s) just before calling \e wl_display_dispatch())
 
  \param ha The event handler function to register
  \param data User data to include on each call

src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx

@@ -1335,7 +1335,11 @@ static const struct wl_registry_listener registry_listener = {
 };
 
 
+extern int fl_send_system_handlers(void *);
+
+
 static void wayland_socket_callback(int fd, struct wl_display *display) {
+  if (fl_send_system_handlers(NULL)) return;
   struct pollfd fds = (struct pollfd) { fd, POLLIN, 0 };
   do {
     if (wl_display_dispatch(display) == -1) {