Adds an XEMBED client, such as a Plug, to the Socket .
The client may be in the same process or in a different process.
To embed a Plug in a Socket, you can either create the Plug with `gtk_plug_new (0)`, call get_id to get the window ID of the plug, and then pass that to the add_id, or you can call get_id to get the window ID for the socket, and call Plug passing in that ID.
The Socket must have already be added into a toplevel window before you can make this call.
| this |
a Socket |
| window |
the Window of a client participating in the XEMBED protocol. |