[pvr] adds support for hiding groups

As the title says this adds support to control the visibility of channel groups. A hidden group is not shown within the channel group selection dialog nor if you switch between groups (previous/next).

PS: yep it's possible to hide the "All channels" group as every other user-defined group ;)

I'm thinking `includeHidden` makes more sense, to me `filterHidden` sounds like "show only hidden groups".