MovieList: Add groupwise sort mechanism and set it to be default.

"Groupwise" sorts recordings descending by date and other items
ascending by name.

My wife was complaining that her MP3s were always being show in a weird
order, and that she had to change it for each folder again and again.
The groupwise sort will do what she expects: In a directory with just
recordings, they'll be ordered by date, and a directories containing
MP3s or DVDs are automatically ordered by name.

For lack of a better name, it's currently called "default" (suggestions
welcome). It also needs an nifty icon for skins too.