Update KContentRenderer mixin to better support custom completion criteria

[rtibbles]

• Add timeSpent as an optional prop for the KContentRenderer mixin
• Add duration as nullable prop. If prop is null, then renderer must define 'durationDefault' data item to set a default value.
• Add reusable progress by time spent computed prop to KContentRenderer mixin
  • Simplifying audio and video progress tracking by just using clock time instead of media track time
  • Allowing for unified time based progress tracking for any content type, based on duration or durationDefault
• Add prop that presents a Boolean for 'always use duration based progress tracking' - forceTimeBasedProgress - which reads from the contentnode options force_time_based_progress flag.

[rtibbles]

Fixed in #224