Allows database plugins to use the EventThread, e.g. for CURL
integration.

Our local EventLoop instance was never run; it was not usable.

Implementations shall use exceptions instead.

There's no point in passing a LightSong reference here; the callee is
interested only in the URI.

The old struct config_param remains only for top-level string options.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Simplify the definition of many build options.

Decouples db/update/Remove.cpp from global variables.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.