The "bool" type is built-in.

When visiting a song, GetSong() was called, but this object was never
returned by calling ReturnSong().  This patch locks the database only
once in Visit() and passes the original song object to the visitor,
avoiding the copy.

Auto-recover from soft errors.

Allow the plugin to allocate the GetSong() return value.

Fixes wrong object URIs with duplicate base names.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Let the C++ compiler take care for freeing the objects safely.

Fixes assertion failures in the Directory library.

Implementation incomplete, but sort-of-works.  DumpDatabase works, but
MPD is still hard-coded on the "simple" plugin.

Let the database plugin do the match.

Remove GLib dependency from some headers.

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.

Reduce the number of lock/unlock cycles, and make database handling
safer.

Random access is not needed, and a linked list is easier to manage: we
don't need to (re-)allocate the pointer array.

Pass only the "name" to a directory, instead of the full (relative)
path.