Added a new functionality to the QUI Cron system for cleaning up cron history. These changes
include:
- An additional cron task defined in `cron.xml` for housekeeping the cron history.
- A new function called `cleanupCronHistory(array $params, Manager $CronManager)` in
`QuiqqerCrons.php` that removes old cron history entries which are older than a defined amount of
weeks.
- Added a new field and additional indexes in the `cron_history` table in `database.xml` to support
the new feature.
- New localized messages are added for the cron cleanup task 'title' and 'description' in
`locale.xml`.

Related: #59