TextBlock style update 

Hi all,

This one passes tests and I ran through a few samples and it seems to behave as expected!

I did make a bunch of changes that I'll comment on inline which should be reviewed. I'm pretty sure this can be improved in a couple of places but I'm not sure how.

@jasonrclark I'm not sure how this should mix with #832. Based on what I see I think they should play nicely.
I'm guessing there's a better way to avoid repeating the `font` and `fill` defaults here, but I'm not sure how to do it (I'm still a little lost on how to scope something like that).