Access to secondary databases

Created by: ederag

There is a need to access other hamster databases as well, not only the main hamster.db, e.g. for merging a foreign database into the main one (#340 (closed)). As recently discussed, this is not possible using the org.gnome.Hamster D-Bus service, which is tied to the main database.

One possibility:

1. add an option (--no-dbus ?) to hamster-cli to bypass D-Bus and use db.Storage directly.
2. add an option (--db-file=<filename>) to hamster-cli.
3. Edit: Still not fully sorted out, but the following one was probably being over-cautious, as correctly indicated by @GeraldJansen below. Of course this access is risky (there is no central server to avoid simultaneous write access in this case), so if the server is running, it should be forbidden to access the server database. (that one will remain hard-coded)

Still thinking, open to discussion.