On the 'moves-tracking-2' branch: add an experimental new editor design.

* subversion/include/private/svn_editor3.h,
  subversion/libsvn_delta/editor3.c
  New files.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1606748 13f79535-47bb-0310-9956-ffa450edef68