- don't use 10 pt font size for width calculations, when in fact we use

- don't use 10 pt font size for width calculations, when in fact we use
  a 8 pt font (set 8 pt in preferences.xml, too)
- fix vertical alignment of digits in label and ladder (temporary
  solution -- the whole font handling needs to be reviewd and fixed)
- simplify nadir and zenith (they always want to be horizontally centered
  on the ladder lines, no?)
- simplify and abstract label box drawing (no need for stippled side lines)
- align text (more) correctly in label boxes