Add double click gesture on image bubbles to open

src/transcript/layouts/image-bubble-layout.vala

@@ -26,6 +26,7 @@ private class ImageBubbleLayout : BubbleLayout
 {
     public string image_path;
     public bool is_downloaded;
+    public string? attachment_guid;
 
     private Graphene.Size image_size;
     private Gdk.Texture? cached_texture = null;