Store configuration of each database in separate file

[harri]

Storing configuration of all databases in single file makes disruptive upgrades as well as maintenance difficult. It would be better to store each database configuration in separate file in .photovault directory. Since the confiuration file format changes (again) in 0.6.0 this would be a good time to do this change.

The new file format should add also information that what is the minimum version of Photovault that can use the database.