t3lib_treeView is the base class for trees and is always
extended by other classes for specific trees.
The patch adds the abstract keyword to the class declaration.

Change-Id: I5a54f1339ccf84c16671d0dd19bb4bc9d1f5747c
Resolves: #39816
Releases: 6.0
Reviewed-on: http://review.typo3.org/13695
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn