Move local databases into a "databases" folder

All databases (manager.db and nxdrive_UID.db) are located into the $HOME/nuxeo-drive folder.
In order to make things clean, those files should be moved into $HOME/nuxeo-drive/databases.

Several pros:
1. Simpler tree. Cleaner.
2. Sometimes when things go wrong, it is asked the user to delete the $HOME/nuxeo-drive folder to restart on a clean base. Now, and it is the core purpose of this ticket, only the $HOME/nuxeo-drive/databases folder should be removed.