Channel selector buttons now can be used to toggle audio monitoring
for individual channel. One minor bug: the array of channels to monitor
is not updated when the graph is rebuilt. If new processors are added,
the mapping might be wrong.

The SpikeDetector now uses SpikeObjects to pass spike messages,
rather than the GenericProcessor "addEvent" code, as previously.
This makes it easier to distribute spikes to multi-channel electrode
displays, such as StereotrodePlot and TetrodePlot. However, there's
still a problem with setting the axes. "Real" spikes have an issue
with constantly changing the axes, whereas simulated spikes do not.