Class shows all scenes in Unity project and helps switching between them.

ProjectScenes.cs

using UnityEngine;
using UnityEditor;
using System.Collections.Generic;
using UnityEditor.SceneManagement;

/// <summary>
/// Class shows all scenes in project and helps switching between them.
/// </summary>
public class ProjectScenes : EditorWindow
{
    internal class ScenesFolder
    {
        public string path;
        public bool unfolded;
        public Dictionary<string, Scene> scenes;

        public void Draw()
        {
            unfolded = EditorGUILayout.Foldout(unfolded, path);
        }
    }

    internal class Scene
    {
        public string path;
        public string name;

        public void Draw()
        {
            var opened = EditorSceneManager.GetActiveScene().path == path;
            var style = opened ? new GUIStyle(EditorStyles.boldLabel) : new GUIStyle(EditorStyles.label);
            style.margin.left = 20;

            if ((GUILayout.Button(name, style)))
            {
                if(EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
                    EditorSceneManager.OpenScene(path);
            }
        }
    }

    private static Dictionary<string, ScenesFolder> sceneFolders;
    private static Vector2 scrollPosition;

    [MenuItem("Window/Project Scenes")]
    static void Init()
    {
        var window = GetWindow(typeof(ProjectScenes));
        window.titleContent = new GUIContent("Project Scenes");
        window.Show();        

        if (sceneFolders == null)
            sceneFolders = new Dictionary<string, ScenesFolder>();
    }

    void OnGUI()
    {
        EditorGUILayout.BeginVertical(EditorStyles.inspectorDefaultMargins);
        EditorGUILayout.LabelField("Scenes in project:");

        scrollPosition = EditorGUILayout.BeginScrollView(scrollPosition);

        string[] scenesGUIDs = AssetDatabase.FindAssets("t:Scene");
        string previousPath = "";
        for (int i = 0; i < scenesGUIDs.Length; i++)
        {
            var scenePath = AssetDatabase.GUIDToAssetPath(scenesGUIDs[i]);
            var folderPath = scenePath.Substring(0, scenePath.LastIndexOf("/"));

            ScenesFolder folder = null;
            if(!sceneFolders.TryGetValue(folderPath, out folder))
            {
                folder = new ScenesFolder() { path = folderPath, scenes = new Dictionary<string, Scene>() };
                sceneFolders.Add(folderPath, folder);
            }

            Scene scene = null;
            if (!folder.scenes.TryGetValue(scenePath, out scene))
            {
                scene = new Scene() { path = scenePath, name = scenePath.Substring(scenePath.LastIndexOf("/") + 1) };
                folder.scenes.Add(scenePath, scene);
            }

            if (previousPath != folder.path)
                folder.Draw();

            if(folder.unfolded)
                scene.Draw();

            previousPath = folder.path;
        }

        EditorGUILayout.EndScrollView();
        EditorGUILayout.EndVertical();
    }
}