QFileSystemWatcher does not succeed on non-existent paths

Watching a nonexistent path should be possible, as this is something that is quite often needed, and we should do this properly in the place where we do other filesystem monitoring.

No platforms that I know of support notifications of created paths, so we'd probably need to use software polling on most. QTBUG-24862 contains information about creating a 'software fallback' mechanism that might be useful for this case.