setCandidates() has too many arguments and setting the orientation
isn't particularly related with it. It might also be useful to switch
orientation without changing the candidates.
https://bugzilla.gnome.org/show_bug.cgi?id=691902
Make it look more like the mockups.
In order to do that we stop using PopupMenu and friends as it doesn't
really buy us anything and just makes it more cumbersome to add the
style classes we need.
https://bugzilla.gnome.org/show_bug.cgi?id=691902
