Lottie playback controls

[kirb]

Hi there! We found the Lottie control really useful to our project, thanks for maintaining it!

For our use case, we needed the ability to pause, resume, and skip through the animation, and also to have callbacks fired when the animation stops or loops.

This adds the following APIs:

• Start() and Stop() - existing methods, now made public
• Pause() and Resume() - same as above but retains the current frame
• SeekToFrame(float frame) - skip to an exact Lottie frame number
• SeekToProgress(float progress) - skip to frame by progress percentage value
• AutoPlay - can be used to prevent immediate playback (default true so behavior remains compatible)
• OnAnimationCompleted and OnAnimationCompletedRepetition event handlers - callback on animation stopping at the end, or completing a repetition.