Supplementary fix for #35640, internal node group trees (inside material, lamp, world, texture and scene) still keep their original library pointer when appending, making them uneditable. Clearing the
lib pointer now has been moved inside the id_clear_lib_data function, with an ugly switch statement to handle integrated node trees.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57401 954f8c5b-7b00-dc11-b283-0030488c597c