Cleanup Database interface

[FloEdelmann]

In preparation for #5417, which I started looking into.

[westnordost]

Oh, you are? That's cool!

By the way, there is probably little reason to keep the Database interface when the database dealing stuff has been migrated to that multiplatform-enabled thing. (We'll see)

[FloEdelmann]

there is probably little reason to keep the Database interface

But we will need to keep the AndroidDatabase class (renamed to something better of course) as a wrapper around the multiplatform SQLite library, because that library is quite bare-bones. Is it better to then refer to that class everywhere? Or should we keep the interface and let the class implement it, just like it is now?

[westnordost]

You are probably right about this. The interface of that library is pretty bare-bones and thus it would be less convenient (and more error-prone) to use it directly.