- Use CompilationUnitRewriteOperationsFixCore to define the AST rewrite
  required for the quick fix
- Use helper methods for retrieving the sealed type, subtype, and
  compilation unit for the sealed type
- Instead of 'problem.getProblemArguments()[1]', use the Java element
  name of the sealed type

Signed-off-by: Roland Grunberg <rgrunber@redhat.com>