QIcon::fromTheme is not thread safe, we can't have it done in the future watcher.
We can solve this by returning null in the watcher, and doing fromTheme in the main thread
when the watcher finishes if we don't have an icon.
(offtopic, I once made the obvious patch to Qt, but it was rejected with
the response that QIcon isn't entirely thread safe, and that apparently
meant none of it should be, which I don't fully agree with but whatever...)