deleting a comm now unregisters the comm. unrehistering the comm does not close it anymore
decoupling the comm lifetime from the widget lifetime
comm_manager.comms correspond to the open comms.
unregister and register both take comm as argument
bad call and missing call to unregister_comm
close() already unregisters the comm
del Widget.widgets[seld.model_id] in close rather than in __del__
setting _comm to None
Merge pull request #6216 from SylvainCorlay/comm-unregister

Unregistering comms in Comm Manager