Fixes to multi-folder support.

Hey Max -- I've fixed a few issues I found with the branch:

  * m_database in DirectoryDao was not a reference so it was not an open connection.
  * A transaction was started within a transaction because of the move of the call of TrackDao::verifyTracks call. I just removed the transaction and that fixed things.
  * instr() is not available in many versions of SQLite, I switched to using a LIKE with a wildcard on the right (This is better also because it matches only at the start of the string, not any occurrence).
  * A QSqlQuery was missing m_database provided to its constructor. 
  * Other minor fixups / word-smithing / etc.
  * markTracksAsMixxxDeleted had a non-valid SQL UPDATE query.

Adding and removing and relocating libraries works for me now. I haven't taken a look at the dynamic update issues you were mentioning. I'll try to do that tonight.