Blender Git Loki

Rework library_query foreach looper - add optional recursivity.

This commit:
* Fixes bad handling of 'stop iteration' (by adding a status flag, so that we can actually
stop in helper functions too, and jumping to a finalize label instead of raw return, to
allow propper clean up).
* Adds optional recursion into 'ID tree' - callback can also decide to exclude current id_pp
from recursion. Note that this implies 'readonly', modifying IDs while recursing is not
something we want to support!
* Changes callback signature/expected behavior: return behavior is now handled through flags,
and 'parent' ID of id_pp is also passed (since it may not always be root id anymore).

Used this new recursive behavior in ID preview generation as an example, still needs more testing!