This is mandatory for liboverride resync, since this feature may imply
we have to create linked overrides in libraries, and there may be
several copies of those.
This is also a first step to a more general support of IDmanagement-editing
library data.
Note that this commit should have absolutely no effect on current code,
as the only function allowed to check unique names for linked IDs
currently is `BKE_libblock_management_main_add`, which is unused.
This commit also adds some basic testing for `BKE_id_new_name_validate`.
Bastien Montagne
db15c9d1bd