[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
Minor, but IMO the temporary here isn't much use, other than to have some copying fun.  If you drop it it drops to a 2 line for() (you may need to switch to const_iterator).

Other than this one, squash them down, build test and get it in. :)