This commit finally allows the database update to scan remote files,
e.g. using the smbclient storage plugin.  However, it is not yet
possible to configure that, therefore the feature is not accessible
yet.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

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.