Added hint text when no audio visual block items are added.

Filtration/ViewModels/ItemFilterBlockViewModel.cs

@@ -317,6 +317,12 @@ namespace Filtration.ViewModels
             get { return Block.HasBlockItemOfType<SoundBlockItem>(); }
         }
 
+
+        public bool HasAudioVisualBlockItems
+        {
+            get { return AudioVisualBlockItems.Any(); }
+        }
+
         private void OnSwitchBlockItemsViewCommand()
         {
             AudioVisualBlockItemsGridVisible = !AudioVisualBlockItemsGridVisible;
@@ -459,6 +465,7 @@ namespace Filtration.ViewModels
             RaisePropertyChanged("RegularBlockItems");
             RaisePropertyChanged("SummaryBlockItems");
             RaisePropertyChanged("AudioVisualBlockItems");
+            RaisePropertyChanged("HasAudioVisualBlockItems");
         }
     }
 }

Filtration/Views/ItemFilterBlockView.xaml

@@ -271,7 +271,9 @@
                                     </DataTemplate>
                                 </ItemsControl.ItemTemplate>
                             </ItemsControl>
-
+                            
+                            <TextBlock Grid.Row="1" FontStyle="Italic" Visibility="{Binding HasAudioVisualBlockItems, Converter={StaticResource InverseBooleanVisibilityConverter}}">To change the appearance of this block, add a Text, Background or Border Block Item above.</TextBlock>
+                            
                             <!-- Block Items -->
                             <WrapPanel Grid.Row="1" MaxHeight="200">
                                 <WrapPanel.Resources>