[pvr] go to parent folder after deleting the last item in a subfolder

Currently when the last recording is deleted you're left in an empty ghost directory that is gone once you go back to the parent folder. This changes that so that you can't end up in an empty folder.

I also changed the call to `TriggerRecordingsUpdate` to a simple `Refresh`. If the recording was reported as deleted by the backend there's no need to fetch the recordings again, something which can take time.

@xhaggi @opdenkamp for review
GetObjectCount uses Size() and decrements by one if the first item is ".." so I don't think we need another method.