organize into upm directory structure

DefaultUI/SerializeReferenceInspectorButton.cs

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+#if UNITY_EDITOR
+
+using System;
+using System.Collections.Generic; 
+using UnityEditor;
+using UnityEngine;
+
+public static class SerializeReferenceInspectorButton
+{
+    public static readonly Color SerializedReferenceMenuBackgroundColor = new Color(0.1f, 0.55f, 0.9f, 1f);
+
+    /// Must be drawn before DefaultProperty in order to receive input
+    public static void DrawSelectionButtonForManagedReference(this SerializedProperty property,  Rect position, IEnumerable<Func<Type, bool>> filters = null) =>  
+        property.DrawSelectionButtonForManagedReference(position, SerializedReferenceMenuBackgroundColor, filters);
+    
+    /// Must be drawn before DefaultProperty in order to receive input
+    public static void DrawSelectionButtonForManagedReference(this SerializedProperty property, 
+        Rect position, Color color, IEnumerable<Func<Type, bool>> filters = null)  
+    { 
+  
+        var backgroundColor = color;   
+            
+        var buttonPosition = position;
+        buttonPosition.x += EditorGUIUtility.labelWidth + 1 * EditorGUIUtility.standardVerticalSpacing;
+        buttonPosition.width = position.width - EditorGUIUtility.labelWidth - 1 * EditorGUIUtility.standardVerticalSpacing;
+        buttonPosition.height = EditorGUIUtility.singleLineHeight;
+
+        var storedIndent = EditorGUI.indentLevel;
+        EditorGUI.indentLevel = 0;
+        var storedColor = GUI.backgroundColor;
+        GUI.backgroundColor = backgroundColor; 
+         
+        
+        var names = ManagedReferenceUtility.GetSplitNamesFromTypename(property.managedReferenceFullTypename);
+        var className = string.IsNullOrEmpty(names.ClassName) ? "Null (Assign)" : names.ClassName;
+        var assemblyName = names.AssemblyName;
+        if (GUI.Button(buttonPosition, new GUIContent(className, className + "  ( "+ assemblyName +" )" )))
+            property.ShowContextMenuForManagedReference(buttonPosition, filters);
+        
+        GUI.backgroundColor = storedColor;
+        EditorGUI.indentLevel = storedIndent;
+    }
+}
+
+#endif

DefaultUI/SerializeReferenceInspectorButton.cs.meta

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+fileFormatVersion: 2
+guid: 50b013bc3b4409b438f8781dee4dbbf1
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

DefaultUI/SerializeReferenceInspectorMiddleMouseMenu.cs

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+#if UNITY_EDITOR
+
+using System;
+using System.Collections.Generic;
+using UnityEditor;
+using UnityEngine;
+
+public static class SerializeReferenceInspectorMiddleMouseMenu
+{
+    public static void ShowContextMenuForManagedReferenceOnMouseMiddleButton(this SerializedProperty property,
+        Rect position, IEnumerable<Func<Type, bool>> filters = null)
+    {
+        var e = Event.current;
+        if (e.type != EventType.MouseDown || !position.Contains(e.mousePosition) || e.button != 2) 
+            return;
+        
+        property.ShowContextMenuForManagedReference(filters);
+    } 
+}
+
+#endif

DefaultUI/SerializeReferenceInspectorMiddleMouseMenu.cs.meta

@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: ba51bcbc90077924abfcf504152b4a51
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: