We cannot nuke treestore in readfile's `blo_lib_link_screen_restore()`, because this will
destroy all UI-state data (like opened/closed items, etc.).
Since we cannot know for sure whether an ID pointer from tselem->id is valid here, we
have to ensure they are never invalid, i.e. to always set them to NULL when we delete them.
To do so, this commit uses a similar approach as what already exists for ID references
in WM notifiers - it extends `free_notifier_reference_cb()` to also nullify those IDs in
all outliners.
Note that some ID types are not used(shown) by outliner currently, so `TREESTORE_ID_TYPE` macro
was added, that checks whether an ID is possibly used by outliner. Avoids a few searches
in whole tree whene deleting some IDs.
Reviewers: campbellbarton, sergey
Maniphest Tasks: T44439
Differential Revision: https://developer.blender.org/D1272
f271d85b86