QDataStream/QIODevice transactions

We added transactions to QDataStream and QIODevice in Qt 5.7, time to take a new look.

• we did a poor job of documenting them:
  • they're not mentioned in the QDataStream \class documentation at all
  • (QIODevice contains a paragraph)
  • apart from the resp. tests, there are only the two fortuneclient examples that use it, but they only show start and commit, not abort or rollback.
  • tie QDataStream, QIODevice and QtSql transactions together
    • by a concept?
    • with a documentation overview page?
• There's also just one level of restore points
  • we should check whether we can support nesting better.
• Can we add RAII wrappers around these functions?