Add VRC6 support

2 Pulse channels:
- 8 level pulse duty cycle, DAC mode(just ignores duty cycle)/pulse wave mode, 4 bit volume.
- Furnace support PCM playback in pulse channels with duty cycle ignore mode.

Sawtooth:
- nothing but 6 bit volume (8 bit accumulator in technically) and 12 bit frequency (periodic).

VRC6 instrument:
- 6 bit Volume macro for finer sawtooth volume handling, also 3 bit Duty cycle macro for pulse channels.

Duty, PCM mode command and Duty macro affects for pulse channel only.

src/gui/guiConst.cpp

@@ -165,5 +165,6 @@ const int availableSystems[]={
   DIV_SYSTEM_BUBSYS_WSG,
   DIV_SYSTEM_PET,
   DIV_SYSTEM_VIC20,
+  DIV_SYSTEM_VRC6,
   0 // don't remove this last one!
 };