[BUGFIX] Multiple values not displayed for simple items lists

t3lib_BEfunc::getProcessedValue() is used throughout the Core to
transform values stored as indices into human-readable values.

However for select-type fields based on a list of items
(and not a foreign table) where multiple selections are possible,
t3lib_BEfunc::getProcessedValue() fails to display anything.
This is due to t3lib_BEfunc::getLabelFromItemlist(),
being unable to handle several values.

This patch introduces a new method to avoid breaking compatibility.

Change-Id: I074099cd5fc06a346a86246bdf7620ebdc7b18cb
Resolves: #31374
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/6379
Reviewed-by: Björn Pedersen
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Francois Suter
Tested-by: Francois Suter