[Feature Request] DPCM sample editor UI improvements

[Gumball2415]

From the internal group chat with @nyanpasu64, @N-SPC700, and @eulyderg.

Proposed improvements to the existing DPCM sample editor:

• Option to switch tilt direction (towards level $00 or $7F)
• DPCM waveform inversion (flip every bit in the sample)
• Flip DPCM preview waveform to match wave level intuition
  • Current preview has level $00 at the top edge

[DWW256]

It would also be very nice to be able to edit individual samples (as in 16000 samples/sec) point-and-click style. I'll often find I have to tilt a sample like 20 times to get it to actually go to 0 at the end instead of, say, 2 or 3.