Prefer QThreadPool over QtConcurrent where we don't care about result

QtConcurrent::run()'s return type is marked with [[nodiscard]], which is
not nice to code that just needs to move some tasks to a worker thread.

On the other hand, QThreadPool satisfies our needs too and Qt doesn't
need to construct futures that we won't use.

One remark about the screenshot plugin: the task lambda is mutable so it
cannot be represented using a std::function.