[pvr] always use last played group as current group on startup

This PR adds two new improvements for the PVR section:

* Always use the last played group as current group on startup whether or not you enabled the option 'continue last channel on startup'. With it we'll have no difference in behavior.
* Trigger job to search for missing channel icons also on wake up

Maybe something for a 14.x release?

@opdenkamp, @Jalle19 mind taking a look please