Deactivate fulltextsearch as default

[teertinker]

Currently, a fresh installed JabRef has fulltextsearch option enabled.
Suggestion: Set the default option to deactivated

Explanation:

1. The fulltextsearch option does not seem to be documented in the help files (https://docs.jabref.org/finding-sorting-and-cleaning-entries/search)
2. The fulltextsearch option indexes and searches files beyond the core program and the loaded libraries. A standard setting that is confined to the core of the program is way more intuitive.
3. Searching a large library takes some time and makes the program unresponsive for a short period. Users who are not aware of the feature might think the program is just slow. This happend to me as a long time user of JabRef. I am still stuck at 4.3. due to performance issues in previous versions. After trying the most recent 5.8 I thought there are still perfomance issues, because searching took way to long for my database. I simply was not aware, that the attached files are search as well.

[koppor]

• Disable file search as default
  
• Button state should be persisted across JabRef sessions (preferences!)
• Deactivate full text index as default
  
• On file search button click
  • If full text index is deactivated:
    • Ask user if they want to activate the full text index?
    • If yes: Activate and start indexing
    • If not: button is deactivated
  • Note that we do not intend to support search based on old index

This refs https://github.com/koppor/jabref/issues/96

[ThiloteE]

Metaissue: #8906