Add an option to lazy load room message at initial sync.

Reduces initial sync time. Useful if you joined a lot of rooms and have
a slow homeserver.

src/roomlistmodel.cpp

@@ -42,6 +42,7 @@ void RoomListModel::doAddRoom(Room* r) {
   if (auto* room = r) {
     m_rooms.append(room);
     connectRoomSignals(room);
+    emit roomAdded(room);
   } else {
     qCritical() << "Attempt to add nullptr to the room list";
     Q_ASSERT(false);

src/roomlistmodel.h

@@ -53,6 +53,7 @@ class RoomListModel : public QAbstractListModel {
 
  signals:
   void connectionChanged();
+  void roomAdded(Room* room);
   void newMessage(Room* room);
 };