QCommandLineParser: pluck some low-hanging fruit re: exception safety

Make
  QCommandLineParser::add{Help,Version}Option()
  QCommandLineOption::setDefaultValue()
  QCommandLineOptionPrivate::setNames()
have transaction semantics: either they succeed, or they change nothing.
It's trivial to provide this guarantee, so do it.

Add a test for the surprising property that setDefaultValue("") resets
defaultValues() to an empty QStringList instead of one that contains
the empty string.

Change-Id: I61623019de3c7d2e52c24f42cc2e23ec5fddc4da
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>