SongLoader: new class that merges duplicate code

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

src/SongLoader.hxx

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_SONG_LOADER_HXX
+#define MPD_SONG_LOADER_HXX
+
+#include "Compiler.h"
+
+class Client;
+class DetachedSong;
+class Error;
+
+/**
+ * A utility class that loads a #DetachedSong object by its URI.  If
+ * the URI is an absolute local file, it applies security checks via
+ * Client::AllowFile().  If no #Client pointer was specified, then it
+ * is assumed that all local files are allowed.
+ */
+class SongLoader {
+	const Client *const client;
+
+public:
+	explicit SongLoader(const Client &_client)
+		:client(&_client) {}
+	explicit SongLoader(const Client *_client)
+		:client(_client) {}
+
+	gcc_nonnull_all
+	DetachedSong *LoadSong(const char *uri_utf8, Error &error) const;
+
+private:
+	gcc_nonnull_all
+	DetachedSong *LoadFile(const char *path_utf8, Error &error) const;
+};
+
+#endif