Add "Dirt" plugin

Assets/Scripts/Editor/Dirt/VisibilityMask.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Dirt
+{
+    [Flags]
+    public enum VisibilityMask
+    {
+        ChangedDirtyValues = 1,
+        UnchangedDirtyValues = 2,
+        Exclusions = 4
+    }
+}