Add support for recentlyused: kio to view properties.

Add "recentlyused" to the URL schemes recognised by ViewProperties as
needing special handling.  User-set view properties for the
recentlyused: kio are then saved under view_properties/recentlyused in
dolphin's app data directory.

(Previously, the recentlyused: kio was treated, by default, as if it was
a remote location for the purposes of view properties.  Since it
displays somewhat specialised results (files/directories from various
locations), users might find it useful to set particular view properties
and have them remembered.)