[TASK] Language handling: splitLabels and locallang.php should be removed

The TYPO3 core currently handles three types of language labels:
* splitlabels (very very old): Based on the constant TYPO3_languages, a
  string is split by |. The only use was the loginLabels but this was
  changed in 4.3, so they only act as fallback right now.
* locallang.php (very old): An array for each language in a .php file.
* locallang.xml (currently used): An XML file with labels.

Change-Id: Idd6905776af755cf54258fe6723ca1226816655d
Resolves: #26519
Reviewed-on: http://review.typo3.org/3058
Reviewed-by: Stefan Neufeind
Reviewed-by: Oliver Hader
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Tested-by: Oliver Hader