FullTextSearch: Make check of existing PDFs optional

[koppor]

I have my PDFs on a NAS. Each time, I start JabRef, JabRef indexes for several minutes. But, the index is already there.

I found out that JabRef checks each PDF for modifications (using the time stamp). On a NAS, this is slow. Thus, I thought, it would be a good idea to make this optional.

Status: Currently, the configuration possibility is made, but no real behavior implemented.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.