Fix copy to

[koppor]

The PR #12374 had several issues

• "copy to" menu always enabled (and not only if two libraries shown)
• "copy to" menu showing the current library only
• "copy to" menu not really refreshing on library change

This PR fixes it.

Steps to test

1. Start JabRef
2. Open more than one library
3. Try "Copy to" context menu
4. Change opened libraries (e.g., close one, open another one)
5. Try "Copy to" context menu

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• [/] Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [/] 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.

[subhramit]

Code comments were very useful

[Siedlerchr]

Use Action Helper needs multiple datahases

jabref/jabgui/src/main/java/org/jabref/gui/actions/ActionHelper.java

Lines 41 to 43 in dd823a4

	public static BooleanExpression needsMultipleDatabases(TabPane tabbedPane) {
	return Bindings.size(tabbedPane.getTabs()).greaterThan(1);
	}

[Siedlerchr]

You can directly call subscribe on lists:
stateManager.getOpenDatabases().subscribe( Runnable )

[Siedlerchr]

use existing methods

[trag-bot]

Comment restates what is obvious from the code context and doesn't provide additional value or reasoning. Comments should add new information not derivable from code.