widgets: Fix styling of Add to Playlist dialog

- Lessen row height
- Hide action area
- Add margin around the list
- Add separator between each row

https://bugzilla.gnome.org/show_bug.cgi?id=724627
widgets: Select new playlist after editing is done

https://bugzilla.gnome.org/show_bug.cgi?id=724627
widgets: Dim "New Playlist" item in Add to Playlist dialog

https://bugzilla.gnome.org/show_bug.cgi?id=724627