Improve entry merge dialog (3-way merge)

[tobiasdiez]

• Add option to show only differences
• Emphasize header (e.g. bold and with different background) as in
• Concert to 3-way merge: the two entries to be merged on the left and right, in the mid the final merged entry. The values of the merged entry in the center should be editable (i.e. text fields). Similar to
  
  • Further Inspiration:
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      - http://baymard.com/ecommerce-product-lists/benchmark/page-types/comparison-tool

[koppor]

Should also support to merge keywords and groups (see JabRef#12).

[Siedlerchr]

Request fromt he forum: Should support merging of files. E.g. when I merge two entries with files attached create one entry with both files.

[koppor]

Current state: https://docs.jabref.org/finding-sorting-and-cleaning-entries/mergeentries

The main issue is that we do not show the result in the dialog

[koppor]

When working on that, one could also solve #2961 (see especially #2961 (comment)).