Correct Font weights

[mpospese]

Intro

The Typography.FontWeight enum maps from a font weight to a number (1-999) using the styles found here

We have the values for thin and ultralight swapped. We're also missing a value for extraBlack (950), aka ultraBlack

Task

1. Correct thin and ultraLight definitions (swap values 100 and 200)
2. Add an extraBlack (950) value

Discussion

Need to think about whether we wish to rename any of the existing values. The current heavy for 800 is problematic because heavy traditionally means 900.