make a 7.x-dev version for the core issue queue?

As per the code freeze and attempts to organize and make sense of things, seems like now's a good time to make a 7.x-dev version for the core issue queue so we can properly assign stuff there that's definitely *not* going into D6. We did this with the 6.x-dev around the D5 code freeze, and it seemed to help. Any objections? Otherwise, I'll just do this.

Cheers,
-Derek