Make local and make single user are back for ID template.
Internally these calls got unified, id_make_local and
id_copy are now used to do these operations for all types
that support it. Also reveals that for some ID types the
implementation is still missing.
Further, some small changes:
* unlink_text is now in blenkernel.
* copy_group was implemented.
* ID template now has an open operator again.
* fix preview to not change material reference count,
even if temporary it shows up with threaded preview.
* id_unlink unifies unlink for text, object and group.