Allow the creation of more than one user in graphical installer

[mdhorn]

Is your feature request related to a problem? Please describe.
Currently, the graphical user interface allows me to create a single user at installation time. The text-based installer allows me to create as many as needed.

Describe the solution you'd like
Deploy the same/similar functionality in the graphical installer.

Additional context
This should still remain on the Required Options as creating at least one user, and defining at least one user as an Administrator is a minimum requirement.