Remove workaround that was implemented to counter a crash during the beginning of this project, this issue was long solved but the workaround was never removed. Clearing the last precached VPK name prevents the engine from helping other threads to properly unload existing handles, if at all.
The KeyValues class belongs here. Also reimplemented most loading methods for KeyValues, and adjusted the VPK building code to account for it. Pointers to the engine's implementation of KeyValues have been moved to a separate header ('keyvalues_iface.h'), as this allows external tools code to utilize the standalone KeyValues class implementation. Playlist utilities are completely separated from the KeyValues header; these have nothing to do with KeyValues other than manipulating a global KeyValues object for the playlists, and thus have been named as such and moved to rtech/playlists.
