Mapper: move map_song_detach() to db/DatabaseSong.cxx

Use Storage::MapUTF8() internally, don't use global variables.

src/client/Client.hxx

@@ -39,6 +39,7 @@ class EventLoop;
 class Path;
 struct Partition;
 class Database;
+class Storage;
 
 class Client final : private FullyBufferedSocket, TimeoutMonitor {
 public:
@@ -173,8 +174,12 @@ public:
 	/**
 	 * Wrapper for Instance::GetDatabase().
 	 */
+	gcc_pure
 	const Database *GetDatabase(Error &error) const;
 
+	gcc_pure
+	const Storage *GetStorage() const;
+
 private:
 	/* virtual methods from class BufferedSocket */
 	virtual InputResult OnSocketInput(void *data, size_t length) override;