Tests to do to validate the plugin:

Setup:
- Start without prefermenu.conf file
- Check that all prefered channels are loaded
- Check that all setup parameters are saved and restored properly
- Check that all "Sort By" options are available
- Check that the default order (unordered) is preserved in prefermenu.conf file
- Add one channel, check that it is saved in prefermenu.conf
- Remove one channel, check that it is removed in prefermenu.conf
- Remove one channel in VDR, marked as prefered. Open prefermenu plugin. Check that channel is removed from the prefered channels.

OSD:
- Check that all "Sort By" options are displayed properly
