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.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Avoid GLib.

Obsolete, our logging library doesn't ues GLib anymore.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Deprecated.

Look up top-level config options by enum (= integer), not by name
string.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

Make sure we get a UTF-8 encoded string.