Qt: Fix ML "Add to playlist" adding items multiple times

popupAddToPlaylist() didn't check for the column of the selected model
indexes when adding items. In the case of the tree view, a selected row
meant selected model indexes for all it's columns, so whatever was
selected was added multiple times.

Close #6408
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 4cc9d5770a108434038e7f5a4e1bf9b3aafa3d8e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/playlist/playlist_model.cpp

@@ -1044,6 +1044,10 @@ void PLModel::popupAddToPlaylist()
 
     foreach( QModelIndex currentIndex, current_selection )
     {
+        /* Don't add item multiple times when there are more columns in the model */
+        if( currentIndex.column() )
+            continue;
+
         playlist_item_t *p_item = playlist_ItemGetById( THEPL, itemId( currentIndex ) );
         if( !p_item ) continue;