Add AnimationFinished event to SKLottieView

[mattleibow]

Description of Change

This PR adds an AnimationCompleted event to SKLottieView to indicate when the animation is no longer playing.

This event fires after the IsComplete is set to true and thus means that it only fires after all the repeats are finished. So an infinite repeating animation will never be complete nor will the event fire. But for a X repeat (including 0 repeats) animation, the event will fire after all the repeats have happened.

Bugs Fixed

• Fixes FinishedAnimation equivalent for Lottie animations #161

API Changes

Added:

class SKLottieView {
    event EventHandler AnimationCompleted;
}

Behavioral Changes

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Updated documentation