[SECURITY] XSS in TCE forms

Properly encode field labels that are set via TSConfig.

Fixes: #25356
Releases: 6.0, 4.7, 4.6, 4.5

Change-Id: Ie61322d25c28cf953d3662fbd78febf64a21a970
Security-Bulletin: TYPO3-CORE-SA-2012-004
Reviewed-on: http://review.typo3.org/13771
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader