Refactor saved room management and fix closing room when switching account

Relevant fixes hidden behind some refactoring:
- set m_lastRoomConfig per-user while setting the connection
- fold m_lastSpaceConfig into m_lastRoomConfig
- set current room to empty explicitely when there is no room

src/app/roommanager.h

@@ -353,7 +353,6 @@ private:
     QString m_arg;
     KSharedConfig::Ptr m_config;
     KConfigGroup m_lastRoomConfig;
-    KConfigGroup m_lastSpaceConfig;
 
     RoomListModel *m_roomListModel;
     SortFilterRoomListModel *m_sortFilterRoomListModel;