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/queue/Queue.hxx

@@ -85,6 +85,9 @@ struct Queue {
 	/** play back songs in random order? */
 	bool random = false;
 
+	/** Last loaded playlist */
+	std::string last_loaded_playlist;
+
 	/** random number generator for shuffle and random mode */
 	LazyRandomEngine rand;