add support for , in key gesture.

src/Avalonia.Input/KeyGesture.cs

@@ -14,7 +14,7 @@ namespace Avalonia.Input
     {
         private static readonly Dictionary<string, Key> s_keySynonyms = new Dictionary<string, Key>
         {
-            { "+", Key.OemPlus }, { "-", Key.OemMinus }, { ".", Key.OemPeriod }
+            { "+", Key.OemPlus }, { "-", Key.OemMinus }, { ".", Key.OemPeriod }, { ",", Key.OemComma }
         };
 
         [Obsolete("Use constructor taking KeyModifiers")]
@@ -164,4 +164,4 @@ namespace Avalonia.Input
             }
         }
     }
-}
+}