Incremental fixes for scrollview work

- Fix existing typos and spacing problems
- Get preferred height, not current height, of shadows
- Let shadows overflow don't clamp them when we have too little space
- Remove a now-unecessary stray MAX()
- Fix up scrollview visibility for the pathological case of no child
- Disconnect from adjustments on remove()
- Don't unset the adjustments on the child on remove(), since they
  already existed or were autocreated on add()

  (We should what we are doing and set the adjustments of the
  scrollbars on the child rather than setting the adjustments of
  the child, so we match GTK+'s scrolllable interface, but this
  at least makes it consistent instead of a weird mix.)

https://bugzilla.gnome.org/show_bug.cgi?id=611740