Add support for images in dialogue windows (#19)

- Extend editor nodes with custom DialogueContent data type that holds either image or text
- Extend the dialogue importer to correctly process the new content into updated RuntimeDialogue content
- Update SpeechBubble to be able to display either text or image
- Add a custom property drawer for the DialogueContent to allow easy switching in graph authoring

Co-authored-by: Michal Pikulski <michal@foolhardyhorizons.com>
Reviewed-on: #19

Assets/Scripts/UI/PauseMenu.cs

@@ -7,10 +7,34 @@ namespace UI
 {
     public class PauseMenu : MonoBehaviour
     {
+        private static PauseMenu _instance;
+        private static bool _isQuitting;
+
+        public static PauseMenu Instance
+        {
+            get
+            {
+                if (_instance == null && Application.isPlaying && !_isQuitting)
+                {
+                    _instance = FindAnyObjectByType<PauseMenu>();
+                    if (_instance == null)
+                    {
+                        var go = new GameObject("PauseMenu");
+                        _instance = go.AddComponent<PauseMenu>();
+                        // DontDestroyOnLoad(go);
+                    }
+                }
+                return _instance;
+            }
+        }
+        
         [Header("UI References")]
         [SerializeField] private GameObject pauseMenuPanel;
         [SerializeField] private GameObject pauseButton;
 
+        public event Action OnGamePaused;
+        public event Action OnGameResumed;
+        
         private void Start()
         {
             // Subscribe to scene loaded events
@@ -32,6 +56,11 @@ namespace UI
             }
         }
 
+        void OnApplicationQuit()
+        {
+            _isQuitting = true;
+        }
+        
         /// <summary>
         /// Sets the pause menu game object active or inactive based on the current level
         /// </summary>