Allow unassigned prompt codes to be applied to prompt schedules.

Overall, I use four prompt codes for my programs, and my RBTs follow a prompt hierarchy in the directions; however, there are times when one of my learners requires errorless teaching, in which I need to be able to add more specific prompt codes into the prompt schedule. I have added those codes to the list; however, I have not assigned them to DTT or other categories because I don't want them to pop up as options for all of my programs. My suggestion is to have all prompt codes on the list show when creating a prompt schedule, and not just the ones that have been assigned to DTT, TA, Maintenance, etc.