Access to main database is actually rarely needed, but some custom
'apply' functions do need it (like Collections' overriding of objects or
children collections).
I.E. only enable auto-override for 'active' selected object when making
an override of a linked group. This will ease on auto-override creation,
and you typically do not want to auto-override most objects in the group
anyway (in proxy system, you could only proxyfy one object of the group
anyaway!).
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.
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.
Bastien Montagne