It was working as:
- Search directory starts a new search
- Search engine starts all search providers, each one in its
own thread.
- User changes query
- Search engine stops all search providers
- Searchs providers, since they are in its own thread, cancel
in a unknown time.
- Search directory starts a new search, even before all providers
are finished.
- Search engine is marked as need to restart.
- Search providers finished.
- Search engine emits finished signal, since all search providers
now are stopped.
- Clients doesn't have a way to know if the engine
actually finished searching the current search, or the previous
search that the client asked to stop. That might confuse clients
if they ask for results.
- Search engine restart the search providers without noticing the
client, that thinks that the latest search it started was finished
already.

So to fix this confusion, only report that the engine actually finished
if the engine is not going t...