Custom GUI, instead of zenity. Allows:

* Fewer screens by consolidating multiple prompts.
* Check same password entered second time and don't allow continuing
  if not.
* Password strengh display, and don't allow continuing if password is too
  weak.
* Make --gui password entry fields longer, so user does not feel they
  need to make password short. (zenity does not seem to allow configuring
  this.)
