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/Dialogue/DialogueContent.cs

@@ -0,0 +1,76 @@
+using System;
+using UnityEngine;
+
+namespace Dialogue
+{
+    /// <summary>
+    /// Content type for dialogue entries
+    /// </summary>
+    public enum DialogueContentType
+    {
+        Text,
+        Image
+    }
+
+    /// <summary>
+    /// Wrapper class for dialogue content that can be either text or image
+    /// </summary>
+    [Serializable]
+    public class DialogueContent
+    {
+        [SerializeField] private DialogueContentType _contentType = DialogueContentType.Text;
+        [SerializeField] private string _text = string.Empty;
+        [SerializeField] private Sprite _image = null;
+        
+        /// <summary>
+        /// The type of content this entry contains
+        /// </summary>
+        public DialogueContentType ContentType => _contentType;
+        
+        /// <summary>
+        /// The text content (valid when ContentType is Text)
+        /// </summary>
+        public string Text => _text;
+        
+        /// <summary>
+        /// The image content (valid when ContentType is Image)
+        /// </summary>
+        public Sprite Image => _image;
+        
+        /// <summary>
+        /// Create text content
+        /// </summary>
+        /// <param name="text">The text to display</param>
+        public static DialogueContent CreateText(string text)
+        {
+            return new DialogueContent 
+            { 
+                _contentType = DialogueContentType.Text, 
+                _text = text 
+            };
+        }
+        
+        /// <summary>
+        /// Create image content
+        /// </summary>
+        /// <param name="image">The image to display</param>
+        public static DialogueContent CreateImage(Sprite image)
+        {
+            return new DialogueContent 
+            { 
+                _contentType = DialogueContentType.Image, 
+                _image = image 
+            };
+        }
+        
+        /// <summary>
+        /// Returns a string representation of this content
+        /// </summary>
+        public override string ToString()
+        {
+            return ContentType == DialogueContentType.Text 
+                ? $"Text: {_text}" 
+                : $"Image: {_image?.name ?? "None"}";
+        }
+    }
+}