Introduce Abstract Class inheritance for better code structuring

[nateplusplus]

Problem There are a few very similar functions across the code base which could be abstracted into an inherited base class. One example is the way all options behave - they all default to using the HTML attribute value if available, then check for the JS API, and finally fall back on a default.

Solution Create an abstract class that is inherited by each of the concrete classes: PushIn, PushInScene, PushInComposition, PushInLayer

Additional context https://javascript.info/class-inheritance