Added an extra index-retrieval attempt for the transition offset extension

It has been observed that it's not always possible to retrieve the index of an item if the container is an bare ItemsControl (rather than ListView/ListBox), so this adds an extra level of robustness

Author: Llifon Osian Jones

MaterialDesignThemes.Wpf/Transitions/IndexedItemOffsetMultiplierExtension.cs

@@ -68,6 +68,15 @@ public override object ProvideValue(IServiceProvider serviceProvider)
                 multiplier = itemsControl.Items.IndexOf(element);
             }
 
+            if (multiplier == -1) //still not found, repeat now using datacontext
+            {
+                var frameworkElement = element as FrameworkElement;
+                if (frameworkElement != null)
+                {
+                    multiplier = itemsControl.Items.IndexOf(frameworkElement.DataContext);
+                }
+            }
+
             return multiplier > -1 ? new TimeSpan(Unit.Ticks * multiplier) : TimeSpan.Zero;
         }
     }