Print positions in the list playlists function.

For simple listing this can be done by the client, but for the search case the client can not determine the correct position.

I hope this does not break existing client implementations.

src/playlist/Print.cxx

@@ -17,6 +17,8 @@
 #include "Partition.hxx"
 #include "Instance.hxx"
 #include "PlaylistError.hxx"
+#include <fmt/format.h>
+#include "client/Response.hxx"
 
 static void
 playlist_provider_print(Response &r,
@@ -43,12 +45,16 @@ playlist_provider_print(Response &r,
 
 		if (playlist_check_translate_song(*song, base_uri,
 						  loader) &&
-		    detail)
+		    detail) {
 			song_print_info(r, *song);
-		else
+			r.Fmt(FMT_STRING("Pos: {}\n"), i);
+		}
+		else {
 			/* fallback if no detail was requested or no
 			   detail was available */
 			song_print_uri(r, *song);
+			r.Fmt(FMT_STRING("Pos: {}\n"), i);
+		}
 	}
 }
 
@@ -69,27 +75,35 @@ playlist_provider_search_print(Response &r,
 
 	unsigned skip = start_index;
 	unsigned n = end_index - start_index;
+	unsigned position = 0;
 
 	while ((song = e.NextSong()) != nullptr) {
 		const bool detail = playlist_check_translate_song(*song, base_uri,
 								  loader);
-		if (!filter->Match(static_cast<LightSong>(*song)))
+		if (!filter->Match(static_cast<LightSong>(*song))) {
+			++position;
 			continue;
+		}
 
 		if (skip > 0) {
 			--skip;
+			++position;
 			continue;
 		}
 
-		if (detail)
+		if (detail) {
 			song_print_info(r, *song);
-		else
+			r.Fmt(FMT_STRING("Pos: {}\n"), position);
+		} else {
 			/* fallback if no detail was requested or no
 			   detail was available */
 			song_print_uri(r, *song);
+		}
 
 		if (--n == 0)
 			break;
+
+		++position;
 	}
 }