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

This PR is in preparation for PR #2246 which replace hardcoded
left/right group switching in PVRChannelsOSD.
Removed unused strings from the english po file
Update missing localization strings
Alignment fix for the filter control in the picutres section
Merge pull request #52 from xhaggi/fixes

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