[pvr] fix marking recordings as watched or unwatched

This PR fixes there distinct bugs:

* when a recording was marked as watched/unwatched. This is because the change was done on a copy of the actual recording, not the correct instance.

* after marking something as watched/unwatched, the selected item didn't change to the next one in the list (as is done in other windows)

* a folder containing only watched items wasn't marked as watched until the folder had been entered once

Fixes http://trac.xbmc.org/ticket/15118
doesn't `GetByFileItem()` get the real one, not the fileitem copy? it could (in theory) be gone if the backend removed it or something before your fileitem list in the gui gets updated, resulting in a null return here.