Only show exit warning if running tasks will not be recovered

[btut]

BackgroundTasks have a new flag that indicates whether JabRef will automatically resume/recover this task when relaunched. Such tasks should not interrupt a user shut-down.
I.e. indexing tasks, that will be continued when launching JabRef again.

Fixes #8468

• 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.

[calixtus]

Thx

[tobiasdiez]

Thanks!