[TASK] Move tslib_content classes to own files

Change-Id: I8ba276f4cf2a20fa788972a629b17f02aab97c13
Resolves: #34058
Releases: 4.7
Reviewed-on: http://review.typo3.org/9085
Reviewed-by: Philipp Gampe
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog