Use Units.gu where possible to make scaling better

Using Units.gu instead of fixed values where possible to make the menu look better on non-Tenderloin targets. Still might need some TLC, but it's a good start on N4 already. I left the original font pixelsizes in there fore now as reference, because we might need to revisit the medium, large etc, because there's a lot of difference between medium and large. 
Can we drop those comments about the old pixel size? We can always refer to them in the upstream source or by looking through the git history.