Remove lag when using the 'Search for files' feature in a folder with a large...

Remove lag when using the 'Search for files' feature in a folder with a large number of files (Issue #914)

The lag is due to a lot of emitted events (`row-deleted`), that should be ignored, being processed.

Setting the `active_search` flag to `TRUE` before calling `thunar_list_model_set_folder` will prevent the processing of those events.