[TASK] Clean up XCLASS handling and comments

Clean up some of the comments of the new XCLASS handling, separate
the deprecation handling to an own method and add a NEWS.txt entry.

Change-Id: I16a828e15c8e3d347b8b331ffb554ac42aa82644
Resolves: #36698
Related: #31893
Releases: 6.0
Reviewed-on: http://review.typo3.org/10865
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn