No more shown finished background tasks

[koppor]

JabRef collected all run tasks. This was confusing. This PR fixes this.

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]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.

[koppor]

Follow-up depends on controlsfx/controlsfx#1559

[LoayGhreeb]

@koppor, @calixtus
This PR causes an exception that prevents JabRef from shutting down properly. When I close JabRef while a background task is running, Will asked either to wait for the task to finish or cancel the task and shutdown. If I choose to cancel, an exception is thrown, JabRef window closes, but IntelliJ shows that JabRef is still running.

Exception

Caused by: java.util.NoSuchElementException: java.lang.IndexOutOfBoundsException
	at java.base/java.util.AbstractList$Itr.next(AbstractList.java:379)
	at java.base/java.util.Iterator.forEachRemaining(Iterator.java:133)
	at java.base/java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1939)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762)
	at org.jabref@100.0.0/org.jabref.gui.util.UiTaskExecutor.shutdown(UiTaskExecutor.java:131)
	at org.jabref@100.0.0/org.jabref.gui.JabRefGUI.shutdownThreadPools(JabRefGUI.java:376)
	at org.jabref@100.0.0/org.jabref.gui.JabRefGUI.stop(JabRefGUI.java:366)
	at javafx.graphics@22.0.2/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$10(LauncherImpl.java:858)
	at javafx.graphics@22.0.2/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:483)
	at javafx.graphics@22.0.2/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:456)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:400)
	at javafx.graphics@22.0.2/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:455)
	at javafx.graphics@22.0.2/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
	at javafx.graphics@22.0.2/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
	at javafx.graphics@22.0.2/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:184)
	... 1 more
Caused by: java.lang.IndexOutOfBoundsException
	at javafx.base@22.0.2/javafx.collections.transformation.FilteredList.get(FilteredList.java:169)
	at com.tobiasdiez.easybind@2.2.1-SNAPSHOT/com.tobiasdiez.easybind.MappedList.get(MappedList.java:31)
	at java.base/java.util.AbstractList$Itr.next(AbstractList.java:373)
	... 15 more

I tried to revert this commit, and the issue was resolved.

To reproduce the issue:

• Checkout the Lucene branch (Lucene search #11542).
• Open a library with PDFs.
• If the files are already indexed, use "Rebuild Index."
• Try to close JabRef before indexing is finished.
• Click Yes when prompted.

[LoayGhreeb]

The issue was that canceling a task removes items from the runningBackgroundTasks list since it's a filtered list. Removing items during iteration causes an IndexOutOfBoundsException.

Reverting this line will fix the issue.
I will reopen the PR.