Introduce a Database API that abstracts opening, upgrade and fallback

[Cris Dywan]

We want a Database base API that handles these recurring use cases:

Open the database.
Apply a schema.
Assert if files are missing.
Upgrade to a newer schema if needed.
Look in the correct folders.
Allow easy fallback to another file format eg. session.xbel or speeddial.json when needed as a signal.

I'm thinking this can be a base class that later grows more convenience like throw'ing errors instead of manually checking db.errormsg and status.