Create foundation for defining player-interactable behaviours.

Includes a prefab that is a simple 'pointer' animated sprite set up with the empty interactable behaviour script.

Open Questions:
* Is this the appropriate method for collision detection on a parent object?
* Is there a specific tag assigned to the player for collision detection?
* Unity was making a bit of a fuss over using an abstract class; is there an alternative method?
I was able to get this working, but with a caveat: Either the parent needs to be responsible for informing this object of collisions, or this object needs a way to be automatically passed collision events from the parent.