Define defaults for general backup config options in admin interface

One new screen must display these settings:

general_users
general_anonymize
general_role_assignments
general_user_files
general_activities
general_blocks
general_filters
general_comments
general_userscompletion
general_logs
general_histories

With this characteristics:

• The settings will be stored under the "backup" component (config_plugins).
• All them default to "true" but the last two defaulting to "false".
• One helpstring for describing them will be available in an standard way (from backup.php lang file).
• Paired with each setting it will be one "lock" setting (like in some parts of the gradebook).
• backup_controller->apply_defaults() will call one static method in backup_controller_dbops.class.php (apply_backup_general_config() or so). In charge of loading those config options and put the settings with the same name with the proper values, with the status = LOCKED_BY_CONFIG if necessary.

Confirm everything is working ok, with all dependencies between settings automatically executed and so on.

Surely the list of config options and settings will grow along the time.