[pvr] Enable channel name updates from the client

Fixes http://trac.xbmc.org/ticket/14930

This fixes the above mentioned ticket (that channel name changes on the backend aren't ever reflected in XBMC unless the PVR database is reset). Now the name is updated from the client automagically except when it has been changed through the channel manager. If it is later changed to an empty string it will again be updated by the backend the next time XBMC is restarted.

@xhaggi and @opdenkamp please review.

If in the future we find that we need another "isUserSet" column we should consider changing them all into a single bitflag column which indicates what has been changed. For now though I think this is a good enough solution.