organize into upm directory structure

DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttribute.cs

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

DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttribute.cs.meta

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+fileFormatVersion: 2
+guid: c64e0f68b41b48939c9b98cc29a19b20
+timeCreated: 1579584059

DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttributeDrawer.cs

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+#if UNITY_EDITOR
+
+using UnityEditor; 
+using UnityEngine;
+
+[CustomPropertyDrawer(typeof(SerializeReferenceButtonAttribute))]
+public class SerializeReferenceButtonAttributeDrawer : 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 labelPosition = new Rect(position.x, position.y, position.width, EditorGUIUtility.singleLineHeight);
+        EditorGUI.LabelField(labelPosition, label);    
+         
+        var typeRestrictions = SerializedReferenceUIDefaultTypeRestrictions.GetAllBuiltInTypeRestrictions(fieldInfo);
+        property.DrawSelectionButtonForManagedReference(position, typeRestrictions);
+        
+        EditorGUI.PropertyField(position, property, GUIContent.none, true);
+        
+        EditorGUI.EndProperty(); 
+    } 
+}
+#endif

DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttributeDrawer.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 5a07638044724309b6088c28a11a15af
+timeCreated: 1579584059