Share threads between multiple QFileSystemWatcher instances

It seems a little nasty to me that (now with the exception of
the dnotify engine) each time you create and use a QFileSystemWatcher
instance you also create a thread. Even though you can monitor more than
one path with each QFileSystemWatcher instance, as a programmer I would
naively be tempted just to create one instance for each "thing" I wanted to
monitor and assume that Qt would take care of appropriate sharing/caching
behind the scenes.