notificationDaemon: Match app based on WM_CLASS

Most tray applets won't have an associated window in the same PID. We
need to fall back to the WM_CLASS in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=673761

src/shell-app-system.h

@@ -49,7 +49,8 @@ ShellApp       *shell_app_system_lookup_app_for_path          (ShellAppSystem  *
                                                                const char      *desktop_path);
 ShellApp       *shell_app_system_lookup_heuristic_basename    (ShellAppSystem  *system,
                                                                const char      *id);
-
+ShellApp       *shell_app_system_lookup_wmclass               (ShellAppSystem *system,
+                                                               const char     *wmclass);
 
 GSList         *shell_app_system_get_all                   (ShellAppSystem  *system);