Cache last event

Cache the last room event so that the room list can be sorted on startup

Fixes network/neochat#88 \
BUG: 455042

src/neochatroom.h

@@ -774,6 +774,8 @@ private:
     QCoro::Task<void> doDeleteMessagesByUser(const QString &user, QString reason);
     QCoro::Task<void> doUploadFile(QUrl url, QString body = QString());
 
+    std::unique_ptr<Quotient::RoomEvent> m_cachedEvent;
+
     QString m_chatBoxText;
     QString m_editText;
     QString m_chatBoxReplyId;
@@ -790,6 +792,8 @@ private Q_SLOTS:
     void countChanged();
     void updatePushNotificationState(QString type);
 
+    void cacheLastEvent();
+
 Q_SIGNALS:
     void cachedInputChanged();
     void busyChanged();