organize into upm directory structure

DefaultAttributes/SerializeReferenceMenuAttribute/SerializeReferenceMenuAttribute.cs

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+using System;
+using UnityEngine;
+
+[AttributeUsage(AttributeTargets.Field)]
+public class SerializeReferenceMenuAttribute : PropertyAttribute
+{
+    
+}

DefaultAttributes/SerializeReferenceMenuAttribute/SerializeReferenceMenuAttribute.cs.meta

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+fileFormatVersion: 2
+guid: 4f1c372fa681cf146973df30a0d969fd
+timeCreated: 1579584059

DefaultAttributes/SerializeReferenceMenuAttribute/SerializeReferenceMenuAttributeDrawer.cs

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+#if UNITY_EDITOR
+
+using UnityEditor;
+using UnityEngine;
+
+[CustomPropertyDrawer(typeof(SerializeReferenceMenuAttribute))]
+public class SerializeReferenceMenuAttributeDrawer : PropertyDrawer
+{
+    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
+    {
+        return EditorGUI.GetPropertyHeight(property, true);
+    }
+
+    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
+    {
+        EditorGUI.BeginProperty(position, label, property);
+        
+        var typeRestrictions = SerializedReferenceUIDefaultTypeRestrictions.GetAllBuiltInTypeRestrictions(fieldInfo);
+        property.ShowContextMenuForManagedReferenceOnMouseMiddleButton(position, typeRestrictions);
+        
+        EditorGUI.PropertyField(position, property, true);
+        EditorGUI.EndProperty();
+    }
+}
+#endif
+

DefaultAttributes/SerializeReferenceMenuAttribute/SerializeReferenceMenuAttributeDrawer.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: eaa31b1c39ee9424abe12fa0034d9d1b
+timeCreated: 1579584059