runners/webshortcuts: Provide result for default provider if no other results exist

By connecting to the queryFinished signal, we know when all other runners are done with matching.
After that we can check if there are any matches and if not add a match for the default provider.

This way we have the advantage of only showing a result when it is relevant.
For example, when we search for apps and get results, the default search engine result would not be needed.

FEATURE: 461424