[DB] add createConnection() method

DONE:
- separate connection openig in to new method createConnection from DB
  constructor. Connection is created lazy when it is needed  in
  tmpGetConn() method, so only reading from file is now fast and without
  creating connection on DB.
- remove obsolete calling getGraphs() from GraphManager constructor
- remove obsolete calling getLayouts() from Graph::addLayout()

TODO:
- separate saveLayout from CoreWindow to GraphManager
- in saveLayout when is not saved yet, add saveing it instead of canceling
  method
- in saveing graph or layout add other test than conn.open that cause
  delay and freeze