datablocks at once.
This simplifies remapping task, since you don't have to ensure your
overrides are created in the correct dependency order.
Uses famous LIB_TAG_DOIT to mark IDs to be overridden.
read/write.
See https://developer.blender.org/D2417 for details.
Note that since static overrides rely heavily on RNA, this commit is
essentially invisible from user PoV, more in next commits.
