[FEATURE] Function for correct item label in BE

The function t3lib_befunc::getLabelFromItemList() gets you the label of
a tca field in the BE. This works nicely but ignores changes done with
TsConfig with code like
TCEFORM.<table>.<field>.addItems.<key> = New value or
TCEFORM.<table>.<field>.aldLabels.<ke> = Change me

The new function will handle it all

Change-Id: Ifd99d321943d72ecb4e16c4974aaa8cddf481b2f
Resolves: #39327
Releases: 6.0
Reviewed-on: http://review.typo3.org/13098
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog