Directory cache invalidation needs to be cleaned up
Currently, directory cache is invalidated any time the user leaves the file browser. This causes an unnecessarily long delay when going from player back to file browser - there is no need to invalidate it for this.
The only condition under which it should be invalidated is upon sort option change. Being able to do this is blocked by the options changes I'm working on (currently, the option on-change handler executes for the initial option application, on boot)