Merge pull request #700 from VNNCC/fix/WindowNotificationManager-NotificationPosition-632

Add missing WindowNotificationManager position pseudo classes in style

Author: amwx

src/FluentAvalonia/Styling/ControlThemes/BasicControls/WindowNotificationManagerStyles.axaml

@@ -41,6 +41,11 @@
             <Setter Property="HorizontalAlignment" Value="Right" />
         </Style>
 
+        <Style Selector="^:topcenter /template/ ReversibleStackPanel#PART_Items">
+            <Setter Property="VerticalAlignment" Value="Top" />
+            <Setter Property="HorizontalAlignment" Value="Center" />
+        </Style>
+
         <Style Selector="^:bottomleft /template/ ReversibleStackPanel#PART_Items">
             <Setter Property="ReverseOrder" Value="True" />
             <Setter Property="VerticalAlignment" Value="Bottom" />
@@ -52,5 +57,11 @@
             <Setter Property="VerticalAlignment" Value="Bottom" />
             <Setter Property="HorizontalAlignment" Value="Right" />
         </Style>
+
+        <Style Selector="^:bottomcenter /template/ ReversibleStackPanel#PART_Items">
+            <Setter Property="ReverseOrder" Value="True" />
+            <Setter Property="VerticalAlignment" Value="Bottom" />
+            <Setter Property="HorizontalAlignment" Value="Center" />
+        </Style>
     </ControlTheme>
 </ResourceDictionary>