Make `Sequence` loopable

[djeedai]

This is a sub-issue of #8. Opening this separately because for Sequence the problem is I believe well-defined, and work can move forward. This is not the case for Tracks though, so splitting the issue in two for clarity.

Allow Sequence to be loopable both with TweeningType::Loop (play all child tweenables once, then restart at first one) and with TweeningType::PingPong (play all child tweenables once, then restart from last back to first, then restart from first to last, etc.).