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/gsocketclient.c

@@ -1679,6 +1679,7 @@ g_socket_client_connect_async (GSocketClient       *client,
     data->enumerator = g_socket_connectable_enumerate (connectable);
 
   data->task = g_task_new (client, cancellable, callback, user_data);
+  g_task_set_source_tag (data->task, g_socket_client_connect_async);
   g_task_set_task_data (data->task, data, (GDestroyNotify)g_socket_client_async_connect_data_free);
 
   enumerator_next_async (data);