Restart on the wizard needs to cleanup component left-overs

  - use two different script engine instances
    - one which lives from the beginning -> the controlScriptEngine
    - the other one which will be reset if there are some new repositories
      loaded
  - now the core has a pointer to the gui object, but it should only be
    used by the script engine so a QObject type should be enough
  - engines are deleted as QObject children from the PackageManagerCore
  - registered downloaded archives are removed

Change-Id: I60a4a32fb2e409059839ec11b10c57357454f57a
Reviewed-by: Michal Klocek <michal.klocek@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>