gio: Add source tags to various GTasks constructed in GLib

This makes them easier to identify when debugging and profiling.

This patch was somewhat less than interesting to write.

https://bugzilla.gnome.org/show_bug.cgi?id=767765

gio/gsocketlistener.c

@@ -813,6 +813,7 @@ g_socket_listener_accept_socket_async (GSocketListener     *listener,
   GError *error = NULL;
 
   task = g_task_new (listener, cancellable, callback, user_data);
+  g_task_set_source_tag (task, g_socket_listener_accept_socket_async);
 
   if (!check_listener (listener, &error))
     {