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
(cherry picked from commit f043aa0e)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

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

@@ -655,6 +655,7 @@ 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*/ );
 }