[TrackableEntities.Common] Separate Core API's: TrackableEntities.Common.Core

[tonysneed]

I think we should take core bits out of TrackableEntities.Common and put them into TrackableEntities.Common.Core:

• TrackingState enum
• ITrackable interface

This is for folks who may want to manually set or get tracking state without concern for higher level API's, such as object graph recursion. Those higher level API's can continue to exist in TrackableEntities.Common, so existing references will remain valid, but Common will depend on Common.Core. That way, someone wishing just to manually set or get TrackingState may do so without bringing in stuff they don't need, which seems like a cleaner design.