Found a critical bug, 3 small bugs, and added edit course feture

Note: There is a critical bug that I fixed, when user failed to create a new course, the second time a user want to create will crash the program.

I have tested all the things for edit course, it works as what I expected without bugs. I can create a Jar file at this version without any warning and error. I also updated the document in the help menu for edit course.

Thanks for everyone all the hard work! =)