Fix indexing task progress

[LoayGhreeb]

Closes #11819.
The issue will be resolved if the PR for hiding completed background tasks (#11821) is merged.

However, this PR fixes the issue even if hiding completed tasks isn't implemented. Progress was shown with an empty title and no progress because sometimes background indexing tasks start but have no files to add to the index.
There was no control to show the task to the user after it started executing. This PR added this option.
Calling task.showToUser(true) after executeWith will show the task if it's still running.

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.

[github-actions]

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[calixtus]

Looks good. Thanks.