Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Be consistent with "list" responses.

This enforces the "not nullptr" rule.

Use milliseconds precision to reduce rounding errors.

Enables using range-based "for".

Replaces GLib's GError.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Remove GLib dependency from some headers.

Instead of doing run-time initialisation, use a constant lookup table.