The 'collection' property is flagged PROP_ID_REFCOUNT, so assume the
modifiers foreachIDLink functions should walk with IDWALK_CB_USER
(instead of IDWALK_CB_NOP)?

Otherwise the modifier wont be included as a user for the collection
(e.g. on file read); removing the collection from the modifier will
decrement usercount though (which in worst case scenario makes the
collection orphan and will result in data loss)