Fix #34016: add backwards compatibility for deprecated region.callback_add
python function, this keep addons working. It's better to use the new function
but might as well avoid breaking compatibility here.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54194 954f8c5b-7b00-dc11-b283-0030488c597c