Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Simpler to use than const_cast.

Allow database plugins to announce that they have been modified.

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.

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

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

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.