Message Content Rework

For now everything should look identical. However this moves to using a model for the content of the message and is intended to lay the foundation for improved message content representation, e.g. splitting up a text message in multiple sections and using different delegates for things like code and quotes.

src/linkpreviewer.h

@@ -60,7 +60,7 @@ class LinkPreviewer : public QObject
     Q_PROPERTY(bool empty READ empty NOTIFY emptyChanged)
 
 public:
-    explicit LinkPreviewer(const NeoChatRoom *room = nullptr, const Quotient::RoomMessageEvent *event = nullptr);
+    explicit LinkPreviewer(const NeoChatRoom *room = nullptr, const Quotient::RoomMessageEvent *event = nullptr, QObject *parent = nullptr);
 
     [[nodiscard]] QUrl url() const;
     [[nodiscard]] bool loaded() const;