fix: synchronize questStepPanelList (#1866)

* fix: synchronize questStepPanelList

Since we create/add to it from AWT thread but have to iterate & access its stuff from client thread, this seems like a
reasonable solution

* fix: clear questStepPanelList before adding panels from a new quest

Before this, we kept adding and adding more panels, making the iteration over steps slower and slower

* Make use of CopyOnWriteArray instead

This is automatically safe for iterations (which Collections.synchronizedList is not)
	and it makes more sense for our use case given the rarity of our writes

src/main/java/com/questhelper/panel/QuestOverviewPanel.java

@@ -40,6 +40,7 @@
 import java.awt.event.ItemEvent;
 import java.net.URLEncoder;
 import java.nio.charset.StandardCharsets;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
@@ -94,7 +95,7 @@ public class QuestOverviewPanel extends JPanel
 
 	private final JButton collapseBtn = new JButton();
 
-	private final List<QuestStepPanel> questStepPanelList = new ArrayList<>();
+	private final List<QuestStepPanel> questStepPanelList = new CopyOnWriteArrayList<>();
 
 	public QuestOverviewPanel(QuestHelperPlugin questHelperPlugin, QuestManager questManager)
 	{
@@ -254,6 +255,7 @@ private JPanel makeDropdownPanel(JComboBox dropdown, String name)
 	public void addQuest(QuestHelper quest, boolean isActive)
 	{
 		currentQuest = quest;
+		questStepPanelList.clear();
 
 		List<PanelDetails> steps = quest.getPanels();
 		QuestStep currentStep;