Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

Replaces decoder_plugin_from_suffix().

Instead of using the first DecoderPlugin that supports the suffix, use
the first one that actually implements the "container_scan" method.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.