Channel 2 sound as a tone test.

add: sound initialization/teardown code
add: ch2 sound code.  Has an ear-tweaked bandpass filter.
change: LCD clears white

The overall strategy for sound generation should eventually be revised so that sound is generated as the CPU executes into a back buffer.  Right now, a register "snapshot" is taken every time the sound card asks for a next batch of data, which is incorrect.
Sound: overhaul everything for CH2.  Now much closer to actual hardware implementation with cascading timers; will be easier to factor out into little bits for the individual channels.

change: rewrite most of the sound code
change: hoist out system bus frequency from the CPU and into the MemoryBus; it is needed by the sound processor
Refactor the square wave generator, the length counter and the volume envelope into little classes that can be reused for other channels.

change: refactor sound code as per the above