[TAKS] FrontendUser misses moduleSysDmailNewsletter

The model has the moduleSysDmailHtml and moduleSysDmailCategory
properties but not the moduleSysDmailNewsletter so it isn't
posible to enable dmail for a user.

This changes adds the moduleSysDmailNewsletter and the getter
and setter function.

Change-Id: I08b52d01f423fb9a19a043d1fd48cc8073f09a88
Reviewed-on: https://review.typo3.org/26976
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer