[PVR] add possibility to start liveTV or PVR radio playback via cAction and keymaps.

When called, it first tries to play the last watched channel - if this fails/is not available it uses the first channel of the current channel group.
[PVR] improve kaitoast error messages on PVR playback issues by naming the channel or playbacktype that failed
Merge pull request #2325 from da-anda/instant-livetv

[PVR] add possibility to start liveTV playback via cAction and keymaps.