Cache property-not-found responses in XfconfCache

Some apps (like xfce4-terminal) will query certain properties very often, and those properties are often not changed from their defaults, and thus do not have any value stored in xfconf. If properties are not found, we can cache that state in XfconfCache and avoid xfconfd round-trips. If the property is later set, xfconfd will emit PropertyChanged, and we will update our cache with the newly-set value.