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 f043aa0e11894803f8b0f6f9f940a660d3a36069)
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*/ );
 }