#427 "-add-to-playlist" resets playlist

[Roman Beslik]

1. Add to playlist some file, say $X.
2. Close "smplayer".
3. Run "smplayer".
4. There is $X in smplayer's playlist.
5. Close "smplayer".
6. Run the command "smplayer -add-to-playlist $Y", where $Y is not equal to $X.
7. There is $Y and no $X in smplayer's playlist. There should be $X in smplayer's playlist. The option "-add-to-playlist" does not say that it removes files.