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?
Both these are properties defined below.  I chose to put public / extensible portions up at the top, then Start/Update/Event handlers, then all the misc. supporting properties.