Add alpha slider

[pdil]

There should be an alpha slider in addition to the hue slider to allow user to change transparency of color.

To-Do

• [ ] Create generic "slider" class
  • Name: PDColorPickerSliderView
  • Allow initializer to define what color component the slider represents
  • Define background color or view to display when the transparency is less than 1.0
• [ ] Create generic slider delegate protocol
  • Transmits whatever the changeable value is to the protocol implementer when changed
  • For example:

    protocol PDColorPickerSliderViewDelegate: class {
      func valueChanged(in slider: PDColorPickerSliderView, to newValue: CGFloat)
    }
    

• [ ] Replace PDColorPickerViewController layout code with vertical UIStackView (brightness/saturation slider, hue slider, alpha slider) for easier management
• [ ] Provide property in PDColorPickerViewController to display or hide alpha slider
  • If hidden, default of 1.0 is used
  • Possibly provide ability to set a different default alpha while still hiding the alpha slider?