Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

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.

Use the C++ API.

RTTI adds overhead to the binary, and we don't need that.

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

No more hard-coded "simple" plugin.

Let the caller take care for legacy conversion.

Preparing for non-"simple" database plugins.

Unused.

Allow using the object outside of DatabaseGlue.cxx.

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

New db_get_song() implementation.

Let the caller decide if the current directory should be visited.

Use this struct for db_walk().

First draft, not really pluggable currently - hard-coded to use the
"simple" plugin, and calls several of its internal functions.

The API is very simple currently, all searches are still performed
over the root "directory" object.  Future changes to the API will move
those search implementations into the plugin, to allow more efficient
implementations, or implementations that don't have the whole tree in
memory all the time.

For accessing the child of a directory, one needs X_OK on the
directory.