Timetable Modification: Section meeting times

We can't expect schools to have their timetables 100% complete on the first day of the school-year, as changes are inevitable we need to allow at least some degrees of freedom.

Times for section meetings:

At the moment you can only remove or add new meetings. You can't set their titles. You can't move them. All timetable events even those in the past get modified. IActivityRemoved/Added events are fired.

In the future we want to be able to move a meeting and have calendars, gradebooks, attendance react to that change appropriately. Also we want to fire different events for retroactive change and for change that would only affect the future occurrences of the meeting.