[Suggestion] Add custom items to timetable

Just a non-urgent, friendly suggestion. Don't know about anyone else, but I find myself having to physically modify my timetable after printing it out (or adding ugly scribbles before sending it to friends).

For example, some engineering mods spring 'unofficial' labs which are signed up for via IVLE, or a project meeting that I have to for every week, or maybe some lecture that...has webcast. Stuff like that.

Although it may be going a bit out of scope and becoming more like a full-fledged planner, I think it's helpful to combine timetable and planner together - I for one refer a lot to my timetable, so having personal stuff like weekly runs on it could help me stick to such routines.

The issue names no files, tests, or entry points. Start by locating the timetable implementation and clarify whether recurring personal events, planner-like items, and printed output are in scope. Done requires an agreed feature design and corresponding acceptance criteria before implementation begins.