[Dev] Simplifying biref definition and kind extensions

[Andi Vajda]

> I have a question though -- since this still adds an attribute to the 
> CalendarEventMixin under the hood, do we still have the problem that a Kind 
> can't be modified once instances of it have been created?  You couldn't 
> create a bunch of CalendarEvents in your repository and then later add in 
> 'foreign' (or whatever we're calling them) attributes to that Kind, right?  I 
> realize this is nothing new, I just wanted to verify my understanding.

The core schema sure supports it. You can add an attribute to a Kind at any 
time, even when items of that kind exist already. What is not done at the 
moment is setting a possible initial value for that new attribute on these 
existing items.

Andi..