Add the possibilty of having a priority list with add-if-none merge rules.
Add the possibilty of having a priority list with add-if-none merge rules.

If there is such a list, the priority of the owner of the entity _to which
we're adding_ something is compared to the priority of whatever source
we're currently importing from.

If there is no priority list, the add-if-none rule will work as before.
That is, any source may add data at the specified path.

libris/librisxl,libris/librisxl,libris/librisxl