Adwaita: define standard text styles

It's about time that we started using light and ultra-bold font weights consistently throughout GNOME. This would look sooooo much better!

This would be a great opportunity to standardise on predefined text styles. This will allow us to update the text styling from the theme in future. It will also allow us to adjust text styles for accessibility purposes.

This is an initial schema we could try:

We should probably try and apply these in a small number of cases, to see how they work in practice, before we roll them out more generally. It should be noted that both Microsoft, Apple and Google define a greater number of styles than this, so we might want more ourselves.

cc @jimmac @bertob @snwh