Blender Git Loki

ID Management: Allow unique name check for linked IDs too.

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`.