It is necessary to signal the search engine that we are finished and
that we found something for it to reliably show the results. It would
sometimes work anyway since it is sufficient if any backend signals
completion. However if GtkSearchEngineModel was the only backend
returning results then things would break.
Make GtkSearchHit carry a GFile instead of an uri. Most of the
search engines already have the object around, and converting
to an uri and back is unnecessary extra work.
This search engine reuses the GFileInfo that is already loaded
for the file list, to ensure that hits from the current directory
always appear promptly.
