Add WindowTransparencyLevel enum.

src/Avalonia.Controls/WindowTransparencyLevel.cs

@@ -0,0 +1,20 @@
+namespace Avalonia.Controls
+{
+    public enum WindowTransparencyLevel
+    {
+        /// <summary>
+        /// The window background is Black where nothing is drawn in the window.
+        /// </summary>
+        None,
+
+        /// <summary>
+        /// The window background is Transparent where nothing is drawn in the window.
+        /// </summary>
+        Transparent,
+
+        /// <summary>
+        /// The window background is a blur-behind where nothing is drawn in the window.
+        /// </summary>
+        Blur
+    }
+}