Blender Git Loki

RNA: make `bpy.data.orphans_purge()` return number of deleted datablocks

Sometimes multiple calls to `bpy.data.orphans_purge()` are needed to
delete all orphans, because a call can turn previously-used datablocks
into orphans. Returning the number of deleted datablocks makes it
possible to keep looping until nothing can be deleted any more.

This patch keeps track of deletions in `id_delete()` so that it can be
returned up the call stack.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D9918