pvr: gui improvements for live tv section

This PR changes the way how to access the live tv channels (tv / radio) and channel groups within the gui. The channels (tv / radio) are populate into the same control list, so for skinners it's easier to integrate the channel view.
It's possible switching between the channel types (tv / radio) via a new control button.
To access the channel groups i integrated a select dialog which contains all groups. This is also accessible via a new control button.
The selected group is now also used for the EPG view, so you can easily switching between groups while you stay in epg view.

The needed skin changes are in a second commit.

![screenshot002](https://f.cloud.github.com/assets/1878991/1277185/fd8a6548-2eb1-11e3-8c87-8889cb870e9c.png)