Refactor equalizer rack and quick effect rack.

* Create dedicated EqualizerRack, QuickEffectRack, and
  StandardEffectRack classes.
* Move all per-rack initialization into a method of the sub-class.
* Share common code between EqualizerRack and QuickEffectRack with a
  PerGroupRack parent class.
* Move group formatting methods for rack, chain, and effect to the rack
  sub-type class.
* Keep a strong reference to racks in DlgPrefEq to protect pointer
  usage and eliminate assumptions that the pointer exists.
* Remove filterDepth / filter CO aliases.
Some code safety changes.