There are an unfortunate number of DeprecatedString conversions required
here, but these should all fall away and look much more pretty again
when other places are also ported away from DeprecatedString.
Leaves only the Element IDL interface left :^)
NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.
