Linkpreviewer Improvements

- Have LinkPreviewers stored in NeoChatConnection so that they don't have to be reloaded everytime the MessageContentModel is refreshed
- This means the link is never changed (it will be swiched for a new previewer with the new link)
- LinkPreviewers are stored by URL so they can be re-used by any event with the same URL

BUG: 484927  (because the offending code is ripped out)

src/models/messagecontentmodel.h

@@ -93,9 +93,10 @@ private:
     QList<MessageComponent> m_components;
     void updateComponents(bool isEditing = false);
 
-    LinkPreviewer *m_linkPreviewer = nullptr;
+    QPointer<LinkPreviewer> m_linkPreviewer;
     ItineraryModel *m_itineraryModel = nullptr;
 
+    void updateLinkPreviewer();
     void updateItineraryModel();
     bool m_emptyItinerary = false;
 };