PVRChannelsOSD: added commands for switching channel group (left/right)

XBMC's PR #2246 (https://github.com/xbmc/xbmc/pull/2246) replace
hardcoded left/right group switching with proper action handling.

This PR adds the action handling to the PVR channel osd.
Merge pull request #590 from xhaggi/pvr

PVRChannelsOSD: added commands for switching channel group (left/right)