Decouple some more from GLib.

Allows forward-declaration.

Replaces struct playlist_provider.

Replaces GLib's GError.

Use the C++ API.

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

Merge locate_song_search() and locate_song_match().

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

Improve error reporting and handling.  command.c gets the new function
print_error(), which sends a GError to the client.

Reduce header dependencies.

Optionally allow all local files.

"Insecure" mode is used for printing playlists.

Allow printing remote playlists with the commands "listplaylist" and
"listplaylistinfo".

Memory leak fix.  The input_stream object passed to
playlist_list_open_stream_suffix() must be closed by the caller - this
however never happens in playlist_list_open_path(), because it does
not return it to the caller.

Same for "listplaylistinfo".

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

Renamed all playlist functions to non-CamelCase.