Merge libraries

[koppor]

As researcher, I have created dozens of .bib files, which I want to consolidate into one: I would like to point JabRef do a directory. Then, it recursively crawls the directory for *.bib files. For each found file: Import it in the currently opened library.

For each entry:

• If there is an equal entry, silently ignore it.
• If there is an entry with the same key, do not import it (or open the merge entries dialog, maybe configurable.)
• If there is a duplicate entry (according to our algorithms), there should a) popup the merge entries dialog) or b) do not the entry at all -- may configurable for "silent dropping" - see above
• If the attached file is not stored relatively under the directory of the bib file where it is imported, ask to copy it.

Note that this issue refs https://github.com/JabRef/jabref/issues/160. That issue is about updated paper-bib-files and a main file, whereas this issue here is about merging data of "old" bib files.