[changeset] - improve clf() compatibility

[John W. Eaton]

On 22-Oct-2008, David Bateman wrote:

| 2) The correct solution is to have a means of removing callback 
| functions in the deletefcn of plotyy. So we need a dellistener function 
| that corresponds to the existing addlistener function.. I'm working on a 
| patch along these lines.

The Matlab docs indicate that calling delete on the handle returned by
addlistener should delete the object remove the listener function.  It
looks like Matlab creates a "handle class object" to represent the
listener function.  I know, we don't have "handle class objects".
Maybe we need to fake something for now, but it seems we should at
least use the same interface if possible instead of introducing a new
dellistener function.

jwe