StateFile: use file system API, log in UTF-8

src/StateFile.hxx

@@ -30,6 +30,7 @@ struct Partition;
 
 class StateFile final : private TimeoutMonitor {
 	Path path;
+	std::string path_utf8;
 
 	Partition &partition;
 
@@ -41,7 +42,8 @@ class StateFile final : private TimeoutMonitor {
 		prev_playlist_version;
 
 public:
-	StateFile(Path &&path, Partition &partition, EventLoop &loop);
+	StateFile(Path &&path, const char *path_utf8,
+	          Partition &partition, EventLoop &loop);
 
 	void Read();
 	void Write();