Move FS utility funcs from OFB to a separate file
Add "Reveal Current Document" command

This reveals the current document in the file browser, by expanding directories to reach it, rather than changing the root directory of the browser.
Ensure DisplayName doesn’t return nil

Although I don’t know when it would return nil, issue #649 indicates that it sometimes does. We now fallback on the full URL in this case (and returning ‘«nil»’ if the url itself is nil). This should give a hint about what file system entries would fail to return a display name.
Make OakSavePanel API block-based
Remove unused method
Make DocumentOpenHelper API block-based