Qt4: correct fix for duplicate items in playlist - service-discovery

Hypothesis: The bug stems from a thread concurrency problem. A PLModel::rebuild() happens
just between SD inserting items into the core playlist and Qt playlist receiving item-added callback.
Thus, the items referred to the callback are inserted twice into the Qt playlist.

Solution: check if item already inserted at callback processing.

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

@@ -667,6 +667,9 @@ void PLModel::processItemAppend( const playlist_add_t *p_add )
     PLItem *nodeItem = findById( rootItem, p_add->i_node );
     if( !nodeItem ) return;
 
+		foreach( PLItem *existing, nodeItem->children )
+			if( existing->i_id == p_add->i_item ) { return; }
+
     PL_LOCK;
     p_item = playlist_ItemGetById( p_playlist, p_add->i_item );
     if( !p_item || p_item->i_flags & PLAYLIST_DBL_FLAG ) goto end;