This method adds an XEmbed client specified by window_id to the GtkSocket. The client may be in the same process or in a different process.
You can embed a GtkPlug in a GtkSocket using one of two methods.
Create the GtkPlug, then call get_id() to get the window ID of the plug, and finally pass that to add_id()
Call the get_id() method to get the window ID for the socket, then create the GtkPlug passing in that ID. The GtkSocket must have already be added into a toplevel window before you can make this call. See also: get_id()