Added method to specify location points of animated colors

Animated colours are automatically spaced evenly across the slider. By
specifying locations they can instead be positioned wherever is
required.
Added 4th example slider

Demonstrates how to specify location of animated colors.
Always calculate correct popUpSize for negative values
Moved comment
Keep _valueRange in an instance variable

_valueRange is then only calculated when min or max changes rather than
when calling currentValueOffset.
Set position of animated colors along the slider

Instead of having to set the positions using a scale from 0.0 - 1.0,
the positions are now set using values on the slider. The animation
keyTimes are calculated automatically.
Added comments
Merged branch: ‘color_animation’
Replaced ‘block’ notification with ‘selector’ version