Mockingboard or Phasor cards can be inserted into any slot.
Change Config->Sound to that slots 4+5 to be individually selected for the 3 soundcard types.
Add MockingboardCardManager class to manage multiple cards and mix the sound buffers.
New class for SSI263 (2 instances per MB/Phasor card)
- support 2nd SSI263 at $Cn20 connected to 6522-A (at $Cn00)
Switch from one-shot phoneme playback to ring-buffer
Use a new dedicated ring-buffer (distinct from the AY8910 ring buffer)
Gets rid of: SSI263Thread, the 2 events, and the 64 voices
Support the 2-bit Duration b7:6 of phoneme register for both SC01 & SSI263 (to fix#929)
Updated save-state
Updated IRQ logging (output cycle & 6522 source)
