Qt4: only apply search to inserted items when search isn't empty

This means we don't rebuild() everything with every item when we populate
playlist without search

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

@@ -654,6 +654,7 @@ void PLModel::processItemAppend( int i_item, int i_parent )
     if( newItem->inputItem() == THEMIM->currentInputItem() )
         emit currentIndexChanged( index( newItem, 0 ) );
 
+    if( latestSearch.isEmpty() ) return;
     search( latestSearch, index( rootItem, 0), false /*FIXME*/ );
 }