Major refactor.

Core now formed and contains all useful utility to create custom ui with restrictions.
Many methods was extracted from different classes and moved to core.

Undo is now supported(because unity's undo bug seems to be fixed).

Example now moved into same level separate folder.

Assets/Textus/SerializeReferenceUI/DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttribute.cs

@@ -0,0 +1,8 @@
+using System;
+using UnityEngine;
+
+[AttributeUsage(AttributeTargets.Field)]
+public class SerializeReferenceButtonAttribute : PropertyAttribute
+{
+    
+}

Assets/Textus/SerializeReferenceUI/DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttribute.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: c64e0f68b41b48939c9b98cc29a19b20
+timeCreated: 1579584059

Assets/Textus/SerializeReferenceUI/DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttributeDrawer.cs

@@ -0,0 +1,31 @@
+#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

Assets/Textus/SerializeReferenceUI/DefaultAttributes/SerializeReferenceButtonAttribute/SerializeReferenceButtonAttributeDrawer.cs.meta

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