* src/EditRepository.py: modified to use find_backlink_handles and fixed subtle bug
* src/RelLib/_Source.py: added reporef_list to referent object lists so that backref
are put in reference_map
* test/GrampsDb/GrampsDbBase_Test.py: added unittest for source/reference backlinks
* test/GrampsDb/GrampsDbTestBase.py: added unittest for source/reference backlinks
svn: r5616
the old Event object.
* src/RelLib/_Event.py: Remove external witnesses altogether.
* src/RelLib/_Witness.py: Make even emptier (needed for upgrade only).
* src/RelLib/__init__.py: Expose Witness (needed for upgrade only).
svn: r5595
