gtksearchengine: Add "got_results" argument to ::finished

gtksearchengine: Add "got_results" argument to ::finished

The filechooser tries to figure out whether it got results by poking
the model, but all files might go through the async GFileInfo querying
state.

Make all search engines (and the composite one) just notify about this
fact, so the file chooser can behave appropriately without waiting for
the async operations to finish.