Wayland: Set "class" for each window we create

This will be used by Wayland compositors to match the application ID and
.desktop file to the SDL window(s).

Applications can set the SDL_VIDEO_WAYLAND_WMCLASS environemnt variable
early in the process to override using the binary name as a fallback.

Note that we also support the SDL_VIDEO_X11_WMCLASS in the Wayland
backend so that if a program correctly associated the desktop file with
the window under X11, only a newer SDL would be needed for it to work
under Wayland.

https://bugzilla.libsdl.org/show_bug.cgi?id=3376

src/video/wayland/SDL_waylandwindow.c

@@ -178,6 +178,7 @@ int Wayland_CreateWindow(_THIS, SDL_Window *window)
     wl_surface_set_user_data(data->surface, data);
     data->shell_surface = wl_shell_get_shell_surface(c->shell,
                                                      data->surface);
+    wl_shell_surface_set_class (data->shell_surface, c->classname);
 #ifdef SDL_VIDEO_DRIVER_WAYLAND_QT_TOUCH    
     if (c->surface_extension) {
         data->extended_surface = qt_surface_extension_get_extended_surface(