Add support for opening to Help -> TargetGuideID

- Issue MatterHackers/MCCentral#3515
Support linking to tree nodes in help (use this for "What's new"
link on initial run

PartPreviewWindow/Tabs.cs

@@ -181,6 +181,11 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 		{
 			this.ActiveTabChanged?.Invoke(this, null);
 		}
+
+		internal int GetTabIndex(SimpleTab tab)
+		{
+			return _allTabs.IndexOf(tab);
+		}
 	}
 
 	public class ChromeTabs : SimpleTabs