Store last loaded playlist

Stores the last loaded playlist uri in the queue struct.
Last loaded playlist is set on load command and cleared with the clear command.
The last loaded playlist is printed in the status command and saved / restored from the partition state file.

Resolves issue #938

src/command/PlayerCommands.cxx

@@ -44,6 +44,7 @@
 #define COMMAND_STATUS_MIXRAMPDELAY	"mixrampdelay"
 #define COMMAND_STATUS_AUDIO		"audio"
 #define COMMAND_STATUS_UPDATING_DB	"updating_db"
+#define COMMAND_STATUS_LOADED_PLAYLIST  "lastloadedplaylist"
 
 CommandResult
 handle_play(Client &client, Request args, [[maybe_unused]] Response &r)
@@ -128,7 +129,8 @@ handle_status(Client &client, [[maybe_unused]] Request args, Response &r)
 			 COMMAND_STATUS_PLAYLIST ": {}\n"
 			 COMMAND_STATUS_PLAYLIST_LENGTH ": {}\n"
 			 COMMAND_STATUS_MIXRAMPDB ": {}\n"
-			 COMMAND_STATUS_STATE ": {}\n"),
+			 COMMAND_STATUS_STATE ": {}\n"
+			 COMMAND_STATUS_LOADED_PLAYLIST ": {}\n"),
 	      (unsigned)playlist.GetRepeat(),
 	      (unsigned)playlist.GetRandom(),
 	      SingleToString(playlist.GetSingle()),
@@ -137,7 +139,8 @@ handle_status(Client &client, [[maybe_unused]] Request args, Response &r)
 	      playlist.GetVersion(),
 	      playlist.GetLength(),
 	      pc.GetMixRampDb(),
-	      state);
+	      state,
+	      playlist.GetLastLoadedPlaylist());
 
 	if (pc.GetCrossFade() > FloatDuration::zero())
 		r.Fmt(FMT_STRING(COMMAND_STATUS_CROSSFADE ": {}\n"),