[FEATURE] Allow t3lib as class prefix

Several parts in the Core require class names to start with
"tx_" or "user_", e.g. for working with user functions in a
particular context. The "t3lib_" prefix is simply missing
here...

Change-Id: I577ce61506f2dacb5805493d29296f1b84b46419
Resolves: #36779
Related: #36726
Releases: 6.0
Reviewed-on: http://review.typo3.org/10901
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader