[TASK] Declare t3lib_extobjbase abstract

The base class of sub-sub-modules is always exentended
by specific classes and should be declared abstract.

Additionally, some reformatting of the main comment
header is done.

Change-Id: Ib95005f906a5c7c5fc30ae38ba07f99f9b6b8611
Releases: 6.0
Resolves: #39813
Reviewed-on: http://review.typo3.org/13693
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn