Differences from UISlider

Does not have api to:

• set right and left icons
• set the thumb image
• set the right and left track images (for a custom track)
• set the right (background track) color

Same features:

• set color for thumb via @c thumbColor
• set color of track via @c trackColor

New features:

• making the slider a snap to discrete values via property numberOfDiscreteValues

-accessibilityActivate

Our implementation closely resembles what UISlider does but it's not an exact match. On an accessibilityActivate we move one sixth of the amount between the current value and the midpoint value.