Support symbolic links in the file manager.

We didn't watch the symbolic link itself before, only the final link
target. We are watching the symbolic link and the final link target now.
Ugly hack: We need to work around QTBUG-15522. The inotify and kqueue
based file system watcher engines (Linux + Mac) don't report changes to
symbolic links. The workaround is to use the polling engine *only for
the links themselves*. The only way to force the use of the polling
engine is by using the auto test hook in QFileSystemWatcher.