stored_playlist: moved functions from playlist.c

The two functions clearStoredPlaylist() and addToStoredPlaylist()
don't belong into playlist.c.  clearStoredPlaylist() was a wrapper for
spl_clear(), and is converted into a CPP macro for now.

src/stored_playlist.c

@@ -25,6 +25,7 @@
 #include "ls.h"
 #include "database.h"
 #include "idle.h"
+#include "ack.h"
 #include "os_compat.h"
 
 static ListNode *
@@ -316,6 +317,27 @@ spl_append_song(const char *utf8path, struct song *song)
 	return PLAYLIST_RESULT_SUCCESS;
 }
 
+int addToStoredPlaylist(const char *url, const char *utf8file)
+{
+	struct song *song;
+
+	song = db_get_song(url);
+	if (song)
+		return spl_append_song(utf8file, song);
+
+	if (!isValidRemoteUtf8Url(url))
+		return ACK_ERROR_NO_EXIST;
+
+	song = song_remote_new(url);
+	if (song) {
+		int ret = spl_append_song(utf8file, song);
+		song_free(song);
+		return ret;
+	}
+
+	return ACK_ERROR_NO_EXIST;
+}
+
 enum playlist_result
 spl_rename(const char *utf8from, const char *utf8to)
 {