Fix container for group item count still visible if display count is off

[BeeJay28]

Fixes #9972

I moved the setting of the 'hits' style (which contains the styling of the grey container) into the update-callback of the displayGroupCountProperty. That way, whenever said property is updated, it clears all styling of it (mimicking the clearing of the setAll() method, that was used before my change) and adds the 'hits' style if the setting corresponding to the property was switched to true.

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. (too small of a feature, not documented)
• 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. (too small of a feature, not documented)

UI Changes:

[Siedlerchr]

Thanks a lot for the contribution!

[AEgit]

JabRef 5.11--2023-10-17--8ede31a
Windows 10 10.0 amd64
Java 21-internal
JavaFX 20.0.2+3

@Siedlerchr I have a high suspicion that removing this feature has now removed the only visual queue that a specific item belongs to a group. Previously you could see that, because the grey rectangle was coloured green, when an item was selected that belonged to that group. Now, if display count is off (which is required for larger databases for performance reasons) you ca no longer visually see, whether a selected item belongs to a group or not. If my suspicion is correct, please revert this change (it is only a cosmetic one, anyway, but it removes actual functionality - e.g., in old JabRef 3.8.2 group assignment of an item was indicated by the group being underlined - in JabRef 5 it was then the colour of the rectangle that indicated group assignment).