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 waveform icons should be made a bit thicker, but otherwise these look cool.

Whenever files are needed for recording, the RecordNode opens them. When
they are no longer necessary (either recording has stopped, or recording
of a particular channel has stopped), the RecordNode closes them.

If continuous channels are toggled for recording within their respective editors,
a "writeContinuousBuffer" function will be activated within the RecordNode. This
function doesn't do anything yet, but it will make it easy to add diskwriting
capabilities in the near future. Once a stable format for events has been established,
a similar function will be implemented for writing event buffers to disk.

Since every processor now gets asked to enable() prior to the start of
acquisition, it's much simpler to move the steps involved in preparing
for acquisition into these methods. This the processor from beginning
acquisition when prepareToPlay() is called upon initialization.