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.)