start_listener issue on tracks with deleted tracks.

[MathiasSchneider-fr]

Hi, i think i have an issue like this one #105
When i delete a track where i have a listener and then create a new one, i can set up a listener, i always have Error handling OSC message: Observer not connected
I think the listener is not properly deleted when the track is.

I will investigate but python and me...

Many thanks for your work !