If a scroll bar is there we can not use the space --> not for width

* it was taken into account before to calculate the app width,
  which gets confusing when users want to do stuff like:
  `width - 180` and expect it to fit snuggingly next to their
  other slot of width 180. See class-book-adjusted.rb
* fixes #580
Use ruby style method calls instead of Java, we decided on that

* We decided on that back in the day (there should be an issue)
* I left some setters intact, because those take two arguments
  and frankly `@real.size = width, height` looks kind of weird
Use accessors for shell for easier testing and now with tests
Merge pull request #946 from shoes/app-width-and-scroll-bars

Do not count scroll bar into app width

* Changelog: bugfix
* fixes #580