Adds `source/blender/blendloader/intern/versioning_common.cc` and
`versioning_common.h` for version independent versioning functions.
I only placed `do_versions_add_region_if_not_found()` in there for now.
`blo_do_version_old_trackto_to_constraints()` could also be added, but
that's so old, I prefer keeping that in `versioning_legacy.c`.
