Fixes #7016 toggle of special fields does not work for sorted entries

[JavuesZhang]

Fixes #7016
Make a copy of selected entries and change field in the copy so that the sort strategy will not influence the result.

The original version used for-each on the selected entry list but ConcurrentModificationException happened. Then I tried to replace it with normal for loop but the toggle problem still exist, though the exception did not occur any more. By checking the list in each loop I found that the order of the items in this list was changed during the for loop. Not exactly knowing how the sorting strategy is performed, I make a copy of the list and do field change on the copied list. Finally the toggle action performs correctly.

• 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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[k3KAW8Pnf7mkmdSMPHz27]

This looks good to me.

I think there used to be a different open issue related to the list update on permutation in

jabref/src/main/java/org/jabref/gui/LibraryTab.java

Lines 475 to 476 in 4bf895b

	// Add the listener that binds selection to state manager (TODO: should be replaced by proper JavaFX binding as soon as table is implemented in JavaFX)
	mainTable.addSelectionListener(listEvent -> Globals.stateManager.setSelectedEntries(mainTable.getSelectedEntries()));

Perhaps that one is also solved by this PR or a similar strategy. I'll see if I can find it 😃

[k3KAW8Pnf7mkmdSMPHz27]

I'd suggest 'We fixed an issue with toggling special fields for sorted entries.'