[BUGFIX] Labels in extension manager not loaded

The extensionmanager tries to load language labels from the
old extension manager.

Add the needed labels to the extension manager language file and
delete the old file. Simultaneously refactor the language handling
a bit.

Change-Id: I937b0ce53de0bfe56219d676f6d008ae7a5d303c
Resolves: #55449
Releases: 6.3
Reviewed-on: http://review.typo3.org/31790
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>