Improve the 'jump to last read message' button

When loading a room, automatically load messages until the last read
message is loaded #35
Don't show the button if the message is not loaded

src/neochatroom.cpp

@@ -56,6 +56,8 @@ NeoChatRoom::NeoChatRoom(Connection *connection, QString roomId, JoinState joinS
 
         NotificationsManager::instance().postNotification(id(), lastEvent->id(), displayName(), sender->displayname(this), eventToString(*lastEvent), avatar(128));
     });
+
+    connect(this, &Room::aboutToAddHistoricalMessages, this, &NeoChatRoom::readMarkerLoadedChanged);
 }
 
 void NeoChatRoom::uploadFile(const QUrl &url, const QString &body)
@@ -613,3 +615,9 @@ bool NeoChatRoom::canSendState(const QString &eventType) const
 
     return currentPl >= pl;
 }
+
+bool NeoChatRoom::readMarkerLoaded() const
+{
+    const auto it = findInTimeline(readMarkerEventId());
+    return it != timelineEdge();
+}