[pvr] add ability to delete a whole recordings folder

Reference: (http://forum.xbmc.org/showthread.php?tid=195849)

Previously if you wanted to delete all recordings from a folder you had to do it one by one. This adds the Delete context button to directory items too (except to the "All recordings" virtual folder) which recursively deletes everything inside that folder.

ping @xhaggi for review
I thought about that too, will change it.