PVR: initial support for sub channel numbers

Just the basics. Display only and only works with 'using backend channel numbers' enabled (so backend->xbmc only)

I've seen quite a lot of feature requests on the forum and trac for this from ATSC users.

Demo add-on with some demo sub channels can be found here: https://github.com/opdenkamp/xbmc-pvr-addons/tree/subchannels
we should use camel case for the typedef name like we do on other places (ex. PVRChannelNumber)
https://github.com/xbmc/xbmc/blob/master/xbmc/pvr/channels/PVRChannelGroup.h#L52