DetachedSong: fork of struct Song

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

src/PlaylistPrint.cxx

@@ -31,6 +31,7 @@
 #include "Client.hxx"
 #include "InputStream.hxx"
 #include "Song.hxx"
+#include "DetachedSong.hxx"
 #include "fs/Traits.hxx"
 #include "util/Error.hxx"
 #include "thread/Cond.hxx"
@@ -151,7 +152,7 @@ playlist_provider_print(Client &client, const char *uri,
 		? PathTraitsUTF8::GetParent(uri)
 		: std::string(".");
 
-	Song *song;
+	DetachedSong *song;
 	while ((song = e.NextSong()) != nullptr) {
 		song = playlist_check_translate_song(song, base_uri.c_str(),
 						     false);
@@ -163,7 +164,7 @@ playlist_provider_print(Client &client, const char *uri,
 		else
 			song_print_uri(client, *song);
 
-		song->Free();
+		delete song;
 	}
 }