SequenceId

[FMovieSceneSequenceID](API\Runtime\MovieScene\FMovieSceneSequenceID) isn't exposed to Blueprints and we need the full support of a [FMovieSceneSequenceID](API\Runtime\MovieScene\FMovieSceneSequenceID) so we can't just store a USequence* like the scripting layer does - we need to be able to handle the same sequence being in a sequence multiple times (which scripting does not).