Add missing XButtons 1 and 2 to MouseButton

src/Avalonia.Input/PointerEventArgs.cs

@@ -107,7 +107,9 @@ namespace Avalonia.Input
         None,
         Left,
         Right,
-        Middle
+        Middle,
+        XButton1,
+        XButton2
     }
 
     public class PointerPressedEventArgs : PointerEventArgs

src/Avalonia.Input/PointerPoint.cs

@@ -90,6 +90,10 @@ namespace Avalonia.Input
                 return MouseButton.Middle;
             if (kind == PointerUpdateKind.RightButtonPressed || kind == PointerUpdateKind.RightButtonReleased)
                 return MouseButton.Right;
+            if (kind == PointerUpdateKind.XButton1Pressed || kind == PointerUpdateKind.XButton1Released)
+                return MouseButton.XButton1;
+            if (kind == PointerUpdateKind.XButton2Pressed || kind == PointerUpdateKind.XButton2Released)
+                return MouseButton.XButton2;
             return MouseButton.None;
         }
     }