DatabasePlugin: add method VisitUniqueTags()

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

src/DatabaseHelpers.hxx

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2003-2012 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_MEMORY_DATABASE_PLUGIN_HXX
+#define MPD_MEMORY_DATABASE_PLUGIN_HXX
+
+#include "DatabaseVisitor.hxx"
+#include "tag.h"
+#include "gcc.h"
+
+class Database;
+struct DatabaseSelection;
+
+bool
+VisitUniqueTags(const Database &db, const DatabaseSelection &selection,
+		enum tag_type tag_type,
+		VisitString visit_string,
+		GError **error_r);
+
+#endif