[WIP] Enable PDF fulltext search

[koppor]

This PR enables a fulltext search of all linked PDFs.

It is an initial implementation skeleton, where work can be started from.

[Braunch]

Some TODOs I thought about:

• include index initialization in JabRef
  • evaluate when to create/update index
  • evaluate if update method needs to be written (that doesn't create the whole index from scratch)
• sort search results
• use pdf importer from [WIP] Show PDF-Comments in JabRef #1883
• validate the extracted fields in the search result
• include in UI
• Qs stuff, unit/integration tests

[koppor]

There is no person working on it.
We are currently trying to focus on other things. 🔥
We will come back later to this topic, therefore closing adding on-hold label and closing the issue for now.

[tobiasdiez]

Some links that are maybe helpful:

• We want: Extended/fulltext search: index pdfs, reviews ect. (related issue: Advanced PDF features #2263)
• Lucene project: https://github.com/schoenm1/lucene
• http://tockit.sourceforge.net/docco/screenshots.html
• Lucene + pdfbox http://www.programming-free.com/2012/11/simple-word-search-in-pdf-files-using.html
• real-live example using javafx http://blog.mirkosertic.de/javastuff/fxdesktopsearch
• Extract references from pdf: https://initiative.eudml.org/EuDmlAnalysisDemo/ or https://github.com/CrossRef/pdfextract

[koppor]

superseded by #2838