Various DSP enhancements

- Update headset to a 2nd order shelf filter, drop allpass delays

  A single shelf filter can give a time delay for low frequencies
  and attenuation for high frequencies yielding a headphone virtualization
  directly.

- Make equalizer out of high shelves.

  Cascading 4 high shelves and one global gain gives me 5 controllable bands
  with passthrough characteristics when not used.

- Set loudness target to 93 %

Also correcting a volume-ramping bug which sometimes caused a slow
adjustment to desired volume because of integer truncation and invalid
round down/up algorithm. This should eliminate cases where song starts
super loud.