Keep only a reference.

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.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

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".

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

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

Reduce heap allocator overhead.

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.