Changes spanning many files.

1) Changed menu system for categories so that it could not leak off of the edge of the program.
2) Fixed a bug where the program would crash upon opening a new course for the second time.
3) Cleared text for category and add new class text fields upon opening those windows so that they did not contain remnants form last time the user was in that menu.
4) Added functionality for removing students.
5) Fixed Exporter so that it exports ghost students as well. Also had to sort ghost students with normal students. They export in alphabetical order.
6) Changed name of window to "GhostGrader."
7) Zach found two bugs. One caused index out of bounds when removing an assignment. One didn't remove a student from the student table immediately upon removing them. Both bugs are fixed.
8) Now shows worth in the assignment window when editing grades (forgot to add this in the last commit).