SequentialTransition and ParallelTransition are the only Transitions that can embed animations. As such, they share a lot of code that represents this behavior. It makes sense to have them extend an abstract common supertype. This type can also be used as the type for `parent` in Animation.

We can also consider having them Implement an interface instead, but this is useful only if there is a way for other classes (including user-defined) that can benefit from it.