Add PlacementMode Center in Popup.

src/Perspex.Controls/PlacementMode.cs

@@ -24,5 +24,10 @@ namespace Perspex.Controls
         /// The popup is placed at the top right of its target.
         /// </summary>
         Right,
+
+        /// <summary>
+        /// The popup is placed at the center of its target.
+        /// </summary>
+        Center
     }
 }