[BUGFIX] Indent with new lines causes invalid output

Adding tabs at the beginning of lines for multi-line labels breaks the output
when the label is used in a preformated environment such as an email template.

Change-Id: I31850fdff437b81f5b07a2e58ac2281d10a8d1c4
Fixes: #31109
Relates: #31035
Releases: 4.6
Reviewed-on: http://review.typo3.org/6062
Reviewed-by: Dominique Feyer
Tested-by: Dominique Feyer
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert