Default emblem for "unknown" file status should be "unsynchronized"

At the moment, the code in ubuntuone-nautilus.c sets "synchronized" emblem hoping to update the emblem when it gets real info from syncdaemon.
Such info might not be received from syncdaemon, i.e. when the file has just appeared in the directory and syncdaemon has not yet finished content hashing (KeyError exception is received instead) or when syncdaemon is not running and dbus calls get timeout during metadata loading.

This was discussed on the mailing list:
Neil Greenwood:
> I agree that "unknown" should start with "not synchronized" rather
> than "synchronized". If possible, my preference would be for an
> "unknown" file to have no emblem.

Thomas Gabelmann:
> Hi, i think the the greyed out syncing icon would be the best
> pictogram for unknown state. I think it speaks for itself.

The fix is trivial in this case.