Add the ability to scroll up in the message search window

This wasn't a thing yet, so the search experience compared to something
like Element Web was much worse. Now you can scroll back as far as any
other client can!

I had to do a bit of refactoring to stop resetting the model all the
time, and append instead. Otherwise, it was quite straightforward to
implement.

src/timeline/models/searchmodel.h

@@ -65,6 +65,9 @@ public:
      */
     Q_INVOKABLE void search();
 
+    void fetchMore(const QModelIndex &parent) override;
+    bool canFetchMore(const QModelIndex &parent) const override;
+
 Q_SIGNALS:
     void searchTextChanged();
     void roomChanged();
@@ -77,8 +80,9 @@ private:
     void setSearching(bool searching);
 
     QString m_searchText;
-    std::optional<Quotient::SearchJob::ResultRoomEvents> m_result = std::nullopt;
+    std::vector<Quotient::SearchJob::Result> m_results;
     Quotient::SearchJob *m_job = nullptr;
     bool m_searching = false;
     QString m_senderId;
+    QString m_nextBatch;
 };