LibraryMonitoredFolder: provide monitoring warning if on network drive in pref view
Fix bug from 20e7fa3 (remove +1px hack on tracktable) and draw top border on library view to match.
Also fix center divider line to match colors.

- Bug caused movement glitching during "Show playlists" slide out animation.
- Also note that this is not quite right for retina, as in retina the track table header view border
is in fact two-toned. See github comment for screenshot