Added Layer Attribute

Editor/LayerAttributeEditor.cs

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+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+using UnityEditor;
+using KairoEngine.Core;
+
+namespace KairoEngine.Core.Editor
+{
+    [CustomPropertyDrawer(typeof(LayerAttribute))]
+    class LayerAttributeEditor : PropertyDrawer
+    {
+
+        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
+        {
+            // One line of  oxygen free code.
+            property.intValue = EditorGUI.LayerField(position, label,  property.intValue);
+        }
+    }
+}

Editor/LayerAttributeEditor.cs.meta

@@ -0,0 +1,11 @@
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Runtime/Attributes/LayerAttribute.cs

@@ -0,0 +1,14 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+namespace KairoEngine.Core
+{
+     /// <summary>
+    /// Attribute to select a single layer.
+    /// </summary>
+    public class LayerAttribute : PropertyAttribute
+    { 
+        // NOTHING - just oxygen.
+    }
+}

Runtime/Attributes/LayerAttribute.cs.meta

@@ -0,0 +1,11 @@
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