Full functions for add/remove assignments

1. When user add assignment, it will display inside the menu, course object, and XML.
2. When user remove assignment, it will remove from the menu, course object.
3. User need to click on save changes when add assignment, otherwise will not save into course object.
4. User need to click File->save to save all the changes in current course
5. Worth only accept Integer
6. User able to change name and worth for assignment, but if user leave blank in either name or worth will not change anything.
7. If user only add worth, but not name, then it will not be saved