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.