[TASK] Improve margin in field select

The margin can be improved.

Change-Id: I5680551fdc7625037d222d627d49a51a49835f26
Resolves: #40858
Releases: 6.0
Reviewed-on: http://review.typo3.org/14536
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Marcus Schwemer
Tested-by: Marcus Schwemer
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn