Select "better" value in MultiMergeEntries dialog

[koppor]

There is org.jabref.logic.bibtex.comparator.plausibility.PlausibilityComparatorFactory#getPlausibilityComparator.

It is currently not used in org.jabref.gui.mergeentries.multiwaymerge.

Should be added.

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This issue is more an "adventure" to analyze the existing code and test and play around etc.