Restructure PropertyModel to Have Separate Entries for Get, Set, and Init

[JasonBock]

Describe the solution you'd like
Right now PropertyModel is doing a bit too much, and it makes it harder to do some code gen parts (e.g. recalculating the MemberIdentifier). Having three properties, Getter, Setter, and Initializer, that has values specific for each accessor, would probably make some things easier to do. Shared information (e.g. Name) would stay on PropertyModel. Maybe call this new type PropertyAccessorModel.