BibMerge: subfield diff/edit, use new BibEdit API
- BibMerge: Add, replace, delete subfield implemented
- BibMerge: Added diff button for subfields
- BibMerge: Updated bibmerge_engine.py to new bibedit_utils API
- BibEdit: Fixed error messages wording.
- BibEdit: Disabled non-supported links to BibMerge.