#14 event metadata

this is not hard, but is probably big. it's basically a new table, on which some extra settings can be set on any event. A list of this might be: * what is the `CACHING_TIME` for _this_ event? so that some events can have bigger caching_time * should it be hidden from calendar? * what is the expected duration of this event? (this might come handy when displaying calendars) * what "color" would you like to give to this event? (calendar appearance) * what "category" would you like to give to this event? (for things like search, or whatever)