Remove "custom on success" logic from Image.

Create a new message element and message layout element type for mod badges.

src/messages/Image.hpp

@@ -68,18 +68,13 @@ public:
     bool operator==(const Image &image) const;
     bool operator!=(const Image &image) const;
 
-    ImagePtr setCustomOnSuccess(
-        std::function<Outcome(ImagePtr, NetworkResult)> onSuccess);
-
 private:
     Image();
     Image(const Url &url, qreal scale);
     Image(qreal scale);
 
-public:
     void setPixmap(const QPixmap &pixmap);
 
-private:
     void actuallyLoad();
 
     Url url_{};
@@ -88,6 +83,5 @@ private:
     bool shouldLoad_{false};
     std::unique_ptr<detail::Frames> frames_{};
     QObject object_{};
-    std::function<Outcome(ImagePtr, NetworkResult)> customOnSuccess_{};
 };
 }  // namespace chatterino