Add `focused` pseudo css class to VirtualizedScrollPane when its content is focused

[JordanMartinez]

Coming from TomasMikula/RichTextFX#507, VirtualizedScrollPane should include a pseudo css class, focused, when its content is focused since the scroll bars are now separated from the content:

import javafx.scene.Node;
import org.fxmisc.flowless.Virtualized;
import org.fxmisc.flowless.VirtualizedScrollPane;

public class VirtualizedScrollPane<T extends Node & Virtualized> extends Region {
    private PseudoClass FOCUSED = PseudoClass.getPseudoClass("focused");

    public VirtualizedScrollPaneExt(T content) {
        super(content);

        content.focusedProperty().addListener((obs, oldVal, newVal) -> {
            pseudoClassStateChanged(FOCUSED, newVal);
        });
    }
}