Use QThreadPool for threading
Use separate thread pool for file saving

With a thread count of 1, to avoid race conditions in File._save_and_rename.
Merge branch 'master' into new-threadpool

Conflicts:
	picard/ui/mainwindow.py
Merge pull request #146 from mwiencek/new-threadpool

Use QThreadPool for threading
Remove a superfluous import of picard.util.queue

It has been removed, so this import made picard unstartable on clean
checkout (old ones most likely has a .pyc file of the queue module lying
around).