Previously, the EventManager stored the calendar events as a raw
JsonArray of objects. Now, we parse the JSON into a Vector<Event>
structure and store that in the EventManager. This makes it easier to
access the events from the outside, as you now don't have to know the
JSON structure anymore.
The user can now save, load, and view calendars. A calendar is made up
of an array of events which are saved in a JSON file. In the future we
should implement the iCalendar standard instead of using a custom
format.
